Normalize Platform.txt to Arduino 1.6.7 master
Normalize Platform.txt to Arduino 1.6.7 master

The original platform.txt that was used as the basis for this port seems
to have been down-revved.  This push replaces platform.txt with a
version originated from the Arduino IDE 1.6.7 release and makes the
needed changes to support ATmegaxxM1 CPUs.

thomasonw/ATmegaxxM1-C1,thomasonw/ATmegaxxM1-C1