at86rf231: replace vtimer_usleep by hwtimer_wait

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.
cpu/stm32f1:board/iot-lab_M3: fix and cleanup for GPIO low level driver
cpu/stm32f1: overwrite HWTIMER_WAIT_OVERHEAD
Merge pull request #1756 from thomaseichinger/welq-fix

cpu/stm32f: boards/iot-lab_M3: cleanup and fixes
cpu/nrf51822: fixed issue with disabled RAM

- on certain devices the second RAM bank might be disabled after
  reset, this fix makes sure its enabled
Merge pull request #1804 from haukepetersen/fix_nrf_ram

cpu/nrf51822: fixed issue with disabled RAM