Notify about dropped packets

Added debug output that warns about packets that are dropped
because the transceiver buffer was full or because the transceiver
failed to notify aiting upper layers.
Merge pull request #1755 from Lotterleben/transceiver_debug_clean

transceiver: notify about dropped packets
board/iot-lab_M3: fixed default terminal device
Merge pull request #1760 from haukepetersen/fix_iotlab_term

board/iot-lab_M3: fixed default terminal device
steps to implement transceiver on atsamr21
Merge branch 'master' of https://github.com/RIOT-OS/RIOT into samr21-transceiver
Changes in 802.15.4 frame format