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

Master

* stm32_hrtim.c: fix burst mode prescaler update

* powerled.h: add fault field to state structure

* stm32f334-disco: add flash mode support for powerled driver + cosmetics

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

 libc and libnx:  When the libraries are built into two libraries, a user space library and a OS space library (as in the PROTECTED and KERNEL build).  Then the user space library must not use the OS internal interfaces; similarly, the OS must avoid using the userspace interfaces so that it does not muck the errno value or create spurious cancellation points.

libc and libnx:  When the libraries are built into two libraries, a user space library and a OS space library (as in the PROTECTED and KERNEL build).  Then the user space library must not use the OS internal interfaces; similarly, the OS must avoid using the userspace interfaces so that it does not muck the errno value or create spurious cancellation points.

configs/z80sim: Fix a naming problem.  Also, don't try to build the serial driver if CONFIG_NFILE_DESCRIPTOR=0.