subsys: net: lib: Add OpenThread platform

OpenThread requires platform definition with standarized API
so we have to add wrappers to make it compatible with Zephyr.
OpenThread is based on autoconf, this requires
more specific CMakeLists.txt which allows to clone specific
commit or point to local copy of openthread.

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