[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).