[uart] add data bits, stop bit and parity config function for uart

only implemented on stm32
[radio] add initial support for Futaba SBUS protocol
[sbus] sbus driver working
[sbus] code cleaning and comments
[sbus] use uart functions instead of macros
[sbus] use gpio helper functions instead of abusing LED
[mcu_periph] sane defines for UART bits defines
[sbus] fix polarity
Merge pull request #485 from paparazzi/sbus

Sbus radio control driver