[imu] GX3 clean up/enhancement
* cleaner code
For RT paparazziL:
* added thread sleep for ChibiOS instead of busy waiting
* more diagnostic information about the imu (unused for standard pprz)
* added mutex on imu data
* proper board configs
* clean up in main_chibios.c
* added FIFO buffer (GX3 doesn't have precise timing unfortunately)