Basic NETSTACK_CONF_RADIO based on kernel 802.15.4

This CL adds a basic NETSTACK_CONF_RADIO implementation based on linux kernel 802.15.4 radio stack, allowing to offload 802.15.4 to kernelspace, e.g. use native contiki in router mode with mrf24 sitting in SPI and being handled by kernel.

The patch is WIP, so your advice on how to clean it up for submission would be much appreciated.