Update import script for version 1.2.0

This version renamed the atmega256rfr2 phy to just atmegarfr2.
Update to Atmel LWM version 1.2.0

This imports (part of) the code from the "Lightweight Mesh Software
Stack v1.2.0" zipfile, as downloaded from the [atmel website][1].

[1]: http://www.atmel.com/tools/lightweight_mesh.aspx