Stm32f3discovery fede

Hi,

This is a request to give Stm32f3discovery support to Paparazzi.

All sensors on the board are supported and they currently give data to paparazzi. The only thing missing is to calibrate and  determine the correct orientation of the sensors with respect to the board.

To compile this it is necessary to apply the changes that I'm sending in another pull-request to libopencm3. Also, be sure to use the proper libopencm3 submodule.