PIC32MX/Z: Partial review of PIC32MZ cp0 register -- need to do more; Also found issues with definitions for PIC32MX -- need to be retested

PIC32MZ: Add file for GPIO interrupt support.  There are issues so configuration is EXPERIMENTAL for now

PIC32MZ: Add beginnings of CPU initialization logic.  Still some bad logic for setting up peripheral clocking

PIC32MZ: Make sure that the microMIPS ISA is selected on all common MIPS32 assembly files when the PIC21MZ is selected

MIPS:  Add some build support for the M14K and also for the PIC32MZ Starter Kit.  Still a long way to go

STM32: Fix for compilation introduced by last backup RAM change.  Not sure it it is correct, be now things do compile

Adds the ability to use the STM32F2 and STM32F4 Battery Backedup SRAM in the file system. With an option to Save Panic context to one of the files. From David Sidrane.

Changes to support fully write protecting the backup domain. N.B. stm32_pwr_enablebkp did not account for the delay from enable to the domain being writable. The KISS solution is a up_udelay. A more complex solution would be a negated write test with restore.  From David Sidrane.