Link layer interfaces

Currently every module above link layer defines their ways to communicate with the transceivers. This results e. g. that the sixlowpan module to this day basically just speaks to cc1100 (and if not, it treets the transceiver like one). To abstract from this I introduced an interface layer similar to the network interfaces in unixoid operating systems. It is basically a wrapper for the transceiver module, with the capability to associate certain upper layer addresses with the a transceiver.