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!
Also: this name is pretty arbitrary. Should other proprietary protocols also use this protocol type? If yes, what good does this information make? The idea of this type definition was to give a higher layer an idea of what to expect from the device or protocol layer (sorry if this was not clear from the documentation) and just saying "this is basically a black box, deal with it" is not very helpful and opens the door to future device driver development just using this flag.

A better name would be `NETDEV_PROTO_NRV24L01X` (X for +), I guess. We have room for at least 65536 different protocols here, so don't be shy to be specific. ;-)