6lowpan changes

Two small changes to the 6lowpan code:
* Added a way to get the RSSI value of the last packet. The RSSI is held in a packetbuf attribute, but was dropped by the 6lowpan layer.
* An improvement to the handling of lost fragments: if a non-fragmented packet arrives when a packet is being reassembled, this is likely a sign that a fragment in the fragmented packet was lost, and we should prioritize the incoming non-fragment instead.