[imu] GX3 clean up

* cleaner code

For RT paparazzi:
* 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)

Note: don't worry about the previous commits, they don't change anything.