Prepend path to dependency info files with sed

The old way was error prone due to it's use of a fixed path file and
confusing.

closes #775
pipe through sed instead

addresses: https://github.com/RIOT-OS/RIOT/pull/778#discussion_r10041955
change new/overlooked Makefiles
remove some newlines
Merge pull request #778 from LudwigOrtmann/dependency_files

Make: Prepend path to dependency info files with sed