BCM2708:  The PiZero configuration now compiles and links cleanly.  Still a few thngs missing internally.

photon: Support SPI1 and SPI3

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

drivers/usbdev:  Move test for NULL pointer before the pointer is deferences.  Noted by Juha Niskanen.

fs/vfs:  Fix after recent changes.  write() was return negative values in errno. Noted by Jussi Kivilinna.

drivers/usbdev: Correct input flow control logic when watermarks are not enabled.  Problem not by and change based on suggestion by Juha Niskanen.

BCM2708: Add system timer register definitions and a partial implementation of the tickless mode timer.

Stm32 rtc small patches

* RTC: canceling an alarm marks it as inactive

* STM32L4, STM32F4, STM32F7 RTC: fix reading alarm value that is more than 24h in future

* STM32F0 RTC: fix backup register count in stm32_rtcc.h

    All other STM32: SHIFTR_SUBFS_MASK was correct in STM32F0 only

* STM32L1: use correct EXTI line definitions

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

Update last commit... Check should really use the definition MMSIZE_MAX which is really the same thing, but guaranteed to be the correct maximum size in any present and future configuration.

If size is greater than (UINT32_MAX - SIZEOF_MM_ALLOCNODE), malloc size can be overflow by MM_ALIGN_UP macro.  For example, if task_create() called with stack_size == -1, up_create_stack() functions allocates SIZEOF_MM_ALLOCNODE bytes for stack.
This can cause data abort in up_stack_color() function.

kthread_create():  Rename kernel_thread() to kthread_create() for better naming consistency with task_create() and kthread_delete().

Initial ADC support for the STM32F33XX

* stm32_adc.h: add JEXTSEL definitions and hrtim trigger configuration

* stm32_adc.c: move STM32F33 ADC logic to a separate file

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