implement basic nRF24L01+ support

You can send and receive data, set output power, data rate, channel, crc length, dynamic payload and auto retransmit parameters.

Auto ack is always enabled (i.e. sender sets both TX and P0 addresses).

By default the nrf is kept at standby-1 mode, you can force a power down after each send/receive using `nrf_enable_powersave()`.