Rename build system module makefiles from "lufa.MODULE.in" to "lufa_MODULE.mk"...

Rename build system module makefiles from "lufa.MODULE.in" to "lufa_MODULE.mk" to prevent makefile from assuming the include is an old style build rule, slowing down the build process.

Prevent the BUILD module from shell-executing multiple processes to determine the avr-size flags to speed up the build, unless the size rule is invoked.

Add warnings and set warnings as errors to the SingleUSBModeTest build test makefile.