- removed neccessity to define empty `DAC_NUMOF 0` for each STM
base board
- adapted all board configs to this
- joined stm32f2 to use common DAC driver
- improved code of DAC driver
* also added a trailing underscore to header guards for consistency
Commit for PR 2623, repairing header file include guards.
This PR is intended to fix the include guards in files under RIOT/boards
SQUASH ME: fix underscore removal overdos
SQUASH ME: consistent macro naming
SQUASH ME: missed that one
SQUASH ME: fixed overdo
SQUASH ME: consistency