[imu] GX3 DMA buffered driver (very efficient) & cleanup 

* 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)

DMA driver:
* uses ChibiOS UART driver which utilizes DMA streams
* very efficient, on Lisa MX CPU usage drops ~10% (non DMA vs DMA driver)
* requires ChibiOS, so it is RT paparazzi only
* minor clean ups in other gx3 files
* added example airframe

Note: don't worry about the previous commits, they don't change anything.