Better "include guards" for the makefiles

Better "include guards" for the makefiles

Checking for ARCH is not good enough, since some subprojects define it.
Ergodox Ez for example. The leads to running the make from
keyboards/ergodox/ez failing. The keyboard makefile will not be included
in that case, and therefore not the CUSTOM_MATRIX either.

Furthermore the output files are read from many different .build
directories, so it doesn't fail deterministically. For example on the
Travis CI the compilation passes, since there's no outdated objects that
needs recompilation.