GNRC adapter for nrfmin

Minimal driver for the NRF51 radio.

void gnrc_netdev_nrfmin_init(void)

Initialize the nrfmin GNRC adapter, also takes care of the nrfmin driver setup.

As we have never more than 1 nrfmin device on a board, we can make some simplifications when it come to allocating device descriptors and adapter data structures -> we do this right in the driver/adapter code, so this function can be called from auto_init as is, without the need for external memory allocation.