Merge pull request #858 from Kijewski/mutex-clutter

core:mutex: remove refactor remnant
Add include paths automatocally for USEMODULES

Application developers use `$(USEMODULES)` in their Makefiles to have
the relevant functionally automagically added to their apps. This even
does basic dependency tracking by means of `Makefile.dep`.

But an important thing is missing: the automatic adding of include
paths. This is inconvenient, error prone, and will hinder the RIOT core
developers in future to change folder structures.