Added a way to get the RSSI of the last received packet
If the 6lowpan layer is currently reassembling a packet, and a non-fragmented packet comes along, this is a sign that one of the fragments were lost (as they normally would arrive back-to-back, in sequence). So we'll grab the non-fragmented packet instead of dropping it.
Merge pull request #456 from adamdunkels/push/6lowpan-changes

6lowpan changes