[gps] add gps_periodic_check to reset fix after timeout

- GpsIsLost should eventually be replaced by isLocal/GlobalCoordinateSystemValid from state interface

closes #637
[stm32] simplifly peripheral clock setup

There is now a convenience function in libopencm3 that makes the clock enable for peripherals easier across different STM families:
e.g.
rcc_periph_clock_enable(RCC_USART1);

closes #642