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.

Add $(MAKEFILE_LIST) as a dependency on build rules that could be altered by changes to the makefiles - if the user alters the project makefile, require that all modules are rebuilt.

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