In #1119 old-style function declarations are exterminated and forbidden.
I missed the functions in `sys/crypto` because they weren't used
throughout the tests/examples before #1124.
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.