core: undefining ENABLE_DEBUG is no longer required
debug: removed superfluous additional debug flag
Merge pull request #2130 from OlegHahm/debug_cleanup

Debug cleanup
doc: periph: various fixes and cleanup

- unify file/module @brief
- distinct @file section more visibly in source
- provide missing @brief's
- move module details/notes from file to module sections
- provide missing macro definition for building
rpl: addr_str not renamed to addr_str_mode
In PR #1404 addr_str was renamed to addr_str_mode,
but at some places it was still referenced as addr_str.
When enabling debug messages, the compiler complains.
Merge pull request #2132 from LudwigOrtmann/doxygen-periph

doc: periph: various fixes and cleanup
doc: periph/dac: fix typo
Merge pull request #2135 from LudwigOrtmann/doxygen-dac

doc: periph/dac: fix typo
Merge pull request #2134 from cgundogan/rpl_addr_str_debug_fix

rpl: addr_str not renamed to addr_str_mode