subsys: net: ip: l2: Add OpenThread L2

Add OpenThread to Zephyrs net stack as data link layer.
OpenThread requires to call process function when an event occurs.
This process function is called from cooperative thread.

Packet conversion and dispaching is implemented in openthread.c
as well as addresses forwarding.

Signed-off-by: Kamil Sroka <kamil.sroka@nordicsemi.no>