drivers/nrf24l01+ : added netdev implementation for nrf24l01+ transceiver

This PR is based on #1704. It contains a connection between the lowlevel part for the nrf24l01+ transceiver-driver and the netdev interface. I assume there will be several deficits, especially because this device fulfills no standard. So let's discuss this!
Should I better let this out? Because this is no real NID but just a number I hold in the `nrf24l01p_t` transceiver struct, if you compare the implementation. I used this for example to store a thread ID, so you can send a message from the on-receive interrupt to a thread which handels the receive procedure with lower priority.