[mcu_periph] Get rid of uart/spi/i2c_pprz.c files, use symlink instead
* these files are necessary for ChibiOS to build  due to its build process
* using relative symlinks fixes this issue, and allows developers to change only one file (uart/i2c/spi.c) so it is easier to maintain
Revert "[mcu_periph] Get rid of uart/spi/i2c_pprz.c files, use symlink instead"

This reverts commit d4c52a5950158f694e86df21965f698725a6e0b5.

ENAC team has a better solution, will be fixed later.
Merge branch 'rt_paparazzi' of git://github.com/paparazzi/paparazzi into rt_paparazzi
[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)
[gx3] DMA buffered GX3 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
[gx3] minor clean up - add error checking for UART DMA port