tests/unittests: merge lib-ringbuffer into core

Fixup for https://github.com/RIOT-OS/RIOT/pull/2017
doc: fix drivers/srf02 documentation
doc: fix drivers/srf08 documentation
Merge pull request #2160 from PeterKietzmann/fix_srf_ultrasonic_doxy_warnings

doc: doxygen fix srf02/srf08 ultrasonic warnings
Merge pull request #2157 from LudwigOrtmann/unittests-lib

tests/unittests: merge lib-ringbuffer into core