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!
The prefix option sets the suffix?!? What I suggested was that `NETDEV_OPT_ADDRESS_TX_PREFIX`/`NETDEV_OPT_ADDRESS_TX_PREFIX` set the 1 byte prefix, while this option sets the suffix. Than there would be no other way then setting the address the prefix+suffix way and this check would not be necessary.