STM32L4 small fixes to RTC

* STM32L4 RTC: init mode was never exited because nested locking in rtc_synchwait() disabled backup domain access

* STM32L4 RTC: use backup register magic value instead of INITS bit

    The INITS (bit 4) of RTC_ISR register cannot be used to reliably
    detect backup domain reset. This is because we can operate our
    device without ever initializing the year field in the RTC calendar
    if our application does not care about correct date being set.

    Hardware also clears the bit when RTC date is set back to year 2000:

    nsh> date -s "Jan 01 00:00:00 2001"
    rtc_dumptime: Setting time:
    rtc_dumptime:   tm: 2001-01-01 00:00:00
    rtc_dumpregs: New time setting:
    rtc_dumpregs:       TR: 00000000
    rtc_dumpregs:       DR: 00012101
    rtc_dumpregs:       CR: 00000000
    rtc_dumpregs:      ISR: 00000037
    ...
    nsh> date -s "Jan 01 00:00:00 2000"
    rtc_dumptime: Setting time:
    rtc_dumptime:   tm: 2000-01-01 00:00:00
    rtc_dumpregs: New time setting:
    rtc_dumpregs:       TR: 00000000
    rtc_dumpregs:       DR: 0000c101
    rtc_dumpregs:       CR: 00000000
    rtc_dumpregs:      ISR: 00000027      <--- Bit 4 went missing!
    ...

    This patch allows us to do:

      stm32l4_pmstop(true);

      /* Stop mode disables HSE/HSI/PLL and wake happens with default system
       * clock. So reconfigure clocks early on Stop mode return.
       */

      stm32l4_clockconfig();

    without stm32l4_clockconfig() doing spurious and harmful backup domain
    reset in rcc_resetbkp().

* STM32L4 RTC: put back the SSR race condition workaround

    ST has confirmed that the issue has not been fixed, and that it applies
    to STM32L4 too (was not in errata sheets due to documentation bug)
    See discussion:

    https://community.st.com/thread/43710-issue-with-rtc-maximum-time-resolution



* STM32F4, STM32L4, STM32F7 RTC: add more CONFIG_RTC_NALARMS > 1 to reduce code size

* STM32L4: rename stm32l4_rtcc.c to stm32l4_rtc.c to better match STM32F7

    Cosmetic changes to comments

* STM32, STM32L4, STM32F7 RTC: stray comment and typos in chip/stm32_rtcc.h

* STM32L4 RTC: change maximum alarm time from 24h to one month

Approved-by: Gregory Nutt <gnutt@nuttx.org>

fs/vfs:  Add new internal OS interface nx_read().  nx_read() is functionally equivalent to read() except that it does not modify the errno variable and it is not a cancellation point.  Changed all references to read() in the OS to nx_read().

net/socket: psock_send/psock_sendto: remove assert check for null psock and buf input pointers.  Removes check as 'psock == NULL' altogether because that checked for later in psock_send and psock_sendto. Change null check for 'buf' so that it is handled same as in recvfrom.c (return -EINVAL instead of
assert).

sched/task:  task_exithook.c fails to link if signals are disabled because was unconditionally trying to send the SIGCHLD signal to the parent in certain configurations.  Noted by Jeongchan Kim.

Adds OS internal function nx_write() which is functionally equivalent to write() except that it does not set the errno variable and do not cause cancellation points.

Adds OS internal functions nx_send(), ns_recv(), and nx_recvfrom() which are functionally equivalent to send(), recv(), and recvfrom() except that they do not set the errno variable and do not cause cancellation points.

fs/vfs:  Change the return value of internal function fs_getfilep(). It no longer sets the errno variable but, rather, returns errors in the same manner as other internal OS functions.

clicker2-stm32: Adds SD card, automount, and syslog file support and fixes a few minor issues

* configs/clicker2-stm32: Moves defines for xbee from clicker2-stm32.h to stm32_xbee.h

* configs/clicker2-stm32: Adds support for uSD click boards and automount support

* configs/clicker2-stm32: Fixes minor guard clause

* clicker2-stm32: Bring-up automounter before MMCSD

* clicker2-stm32: MRF24J40 interrupt should only fire on falling edge.

* clicker2-stm32: Adds file syslog support for logging to file on SD card

Approved-by: Gregory Nutt <gnutt@nuttx.org>

The INA219 is a combined voltage and current sensor that can measure up to 26 volts and a current that depends on an external shunt resistor.  Connection happens via i2c/smbus and the chip features a power supply rail that is independent from the measured voltage, so it can measure low voltages.

This commit adds a driver for this chip.  Right now it measures bus voltage and current, and does not use the internal calibrated current reading, nor the available power measurement.

Fix a few places where there was a semicolon following the 'if' condition, makeing the following logic unconditional.

    Change all calls to mq_receive() and mq_timedreceive() in the OS to calls to nxmq_receive() and nxmq_timedreceive(), making appropriate changes for differences in return values.

    sched/mqueue:  Add nxmq_receive() and mxmq_timedreceive() which are functionally equivalent to the standard mq_receive and mq_timedreceive() except that (1) they do not create cancellation points, and (2) the do not modify the application's errno variable.

    Change all calls to mq_send() and mq_timedsend() in the OS to calls to nxmq_send() and nxmq_timedsend(), making appropriate changes for differences in return values.

    sched/mqueue:  Add internal function nxmq_send() and nxmq_timedsend() that are equivalent to mq_send() and mq_timedsend() except that they do not create cancellation points and do to not modify the errno variable.

    BCM2708:  Add enough infrastructrue (more stubs) to get a clean compilation of the Pi Zero configuration (with many undefined things at link time).

    BCM2708:  Add basic interrupt handling logic

    BCM2708: Add interrupt register definitions.

    BCM2708:  Add irq.h header file

    BCM2708/Pi zero:  bcm_boot.c and bcm_memorymap.h now compile.  Added pizero linker script.

    BCM2708/Pi Zero:  Add Make.defs needed to build.

    arch/arm/include/bcm2708, arch/arm/src/bcm2708, configs/pizero:  Add some basic build and configuration logic.

    configs/pizero:  Add some basic structure of the Rasperry Pi Zero port.

    Created directory configs/pizero.  Nothing there now but a README.txt file.

    Add initial boot.c and memorymap.c files
    Author: Alan Carvalho de Assis <acassis@gmail.com>

    Add AUX/UART/SPI registers definition
    Author: Alan Carvalho de Assis <acassis@gmail.com>

    Pizero GPIO registers
    * Initial commit to add GPIO definitions
    * Add remaining GPIO registers definition
    Alan Carvalho de Assis <acassis@gmail.com>

    BCM2708 memory map:  Add VBASE defintions; fix VCSDRAM address per Alan; move all virtual address to the bottom of the file to avoid confusion -- top is all physical address; bottom is all veritural address.

    Add initial memory map to BCM2708/BCM2835
    Alan Carvalho de Assis <acassis@gmail.com>

Master

* power: Add powerled to Kconfig

* stm32_powerled.c: cosmetics

* stm32_hrtim.c: cosmetics

* stm32/Kconfig: add HRTIM configuration and add DAC external trigger configuration

* stm32f334-disco: Add powerled example configuration

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Stm32, stm32l4 serial patches

* stm32: serial: add interface to get uart_dev_t by USART number, stm32_serial_get_uart

* stm32: serial: do not stop processing input in SW flow-control mode

* stm32l4: serial: do not stop processing input in SW flow-control mode

* stm32l4: serial: suspend serial for Stop mode

Approved-by: Gregory Nutt <gnutt@nuttx.org>

    sched/mqueue:  Rename all private static functions for use the nxmq_ vs. mq_ naming.

    sched/mqueue:  Rename all OS internal functions declared in sched/mqueue/mqueue.h to begin with nxmq_ vs. mq_.  The mq_ prefix is reserved for standard application interfaces.

RNDIS: Use LPWORK for network operations by default