cpu/stm32f: boards/iot-lab_M3: cleanup and intermediate workaround

This fixes a bug where the callback to unlock the thread, put to sleep by
`vtimer_usleep` wouldn't be woken up due to overhead. Using hwtimer_wait
works for now, but this section will be replaced a driver refactor anyway.

see https://github.com/RIOT-OS/RIOT/pull/1752#issuecomment-57532066