[rt_paparazzi] update 0.3.1
Based on @flixr's comments
1) i2c/uart/spi.c renamed and references refactored to i2c/uart/spi_pprz.c
2) fixed doxygen comments, cleaned up code style
3) cleaned ms5611 periph to be more better integrated with ChibiOS
4) ImuEvent moved into main_chibios.c
5) main_chibios.c cleaned
6) supressed warnings for unused arguments in arch functions
7) fixed callback for threads (see example with GPS thread)
8) uart_receive_x macro changed into a function (look at GPS thread)
9) defined SPI_VOLATILE qualifier for SPI volatile buffers