Initial ChibiOS experiment:

- Added ChibiOS git submodule
- Copied ARMCM3-GENERIC-KERNEL demo into chibios_test/
- Modified demo for CM4 with FPU enabled
- Copied STM32F405 linker script and modified to add memory area for CoffeeFS
- Existing piksi_firmware sources built by demo Makefile
- Updated USART DMA ISRs to work with ChibiOS
- Calling init() from main which inits the clocks, FPGA and serial IO
- Tested SBP serial IO - working!
- Updated EXTI1 ISR (from FPGA) to work with ChibiOS
- Tried calling manage_acq() and friends, seems to work for a while then crashes