- new IMU driver structure with state machine (no sleeps in bus thread)
- verify all configured registers and trigger reset on failure
- detect if DIO1 or DIO2 are actually connected for data ready interrupt usage
- don't use CRC-16 on burst transfers except for verified lots
- fmu-v5 debug and stackcheck builds now need to be CONSTRAINED_FLASH to fit
- delete unused holybro_durandal-v1_stackcheck build (also CONSTRAINED_FLASH)
