AS6 version

Change the part in the makefile and the project settings to match the
part you're trying to use.
Merge remote-tracking branch 'origin/AS6_Ext_Makefile'
many xmegas added; cross platform changes

All Xmegas in the Atmel 3.4.2 Toolchain have been added.
Removed automatic execution of sizebefore and sizeafter from the all
target. They were causing cross-platform issues.