Change the build system build module makefiles to have a set of sanity check macros they can call to verify user data. Add additional sanity checks.

Add new ATPROGRAM build system module. Add ATPROGRAM and CPPCHECK modules to all application makefiles.

Add explicit blank checks in the build system for user-set but blank variables that must not be blank.

Add .PHONY targets in all build system modules. Alter BUILD module so that the existence of source files are checked before the build is started.

Clean up CORE build system module, use simple rather than recursive make variables for internal constants and $(shell) derived values.

Remove any variables in the optional build variable list that are marked as mandatory by one or more included build system modules in the CORE build system module. Alter DOXYGEN build system module to by default override the Doxygen configuration file stylesheet and replace it with the LUFA Doxygen stylesheet.

Add EEPROM programming target to the AVRDUDE build system module. Correct message output test for the BUILD build system module. Clean up DOXYGEN build system module logic.