Initial import of low-level driver doxygen groupdef
Fixed spelling errors
Do not assume 0x0 to be an unusable address
Merge pull request #611 from haukepetersen/periph_driver_doc

Introduction of a low-level device driver interface
Merge pull request #628 from Kijewski/fno-delete-null-pointer-checks

Do not assume 0x0 to be an unusable address