make: easify {sys,drivers}/Makefile

Currently you need to add every new sys and driver module into the
respective Makefile. This requires rebasing if another module was merged
in the meantime.

This PR allows you to omit the entry to {sys,drivers}/Makefile, if the
subfolder has the same name as the module name, which should be sensible
in most cases.
Merge pull request #1986 from Kijewski/smaller-sys_makefile

make: easify {sys,drivers}/Makefile