[WIP] Basic 802.15.4 driver code based on mrf24j40

This demo can talk to arduino and to contiki mote, which looks awesome. The code, however, isn't finished yet, I'll clean it up in a few upcoming days.

The primary reason for this driver is to raise code complexity and to see how zinc looks if things are starting to get tricky. So far, passing `timer` around feels a bit cumbersome.

The code doesn't support interrupts yet, as there are no interrupts in hal, that would be next piece of work to do before this PR lands.

Also, this code hardcodes parts of IEEE 802.15.4 MAC, which is a bit unreasonable, but acceptable for the moment. Further improvement should be done to decouple MAC from PHY (more complexity and more challanges).