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
Merge branch 'upstream'
Update PHY_ATMEGA256RFR2 define in config.h

LWM 1.2.0 has generalized the PHY implementation to all RFR2 devices, so
this define has changed to PHY_ATMEGARFR2.