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!
Why is it up here? Setting this value to 8 but leaving the lower values below makes the code harder to read, I think.