drivers: some improvements to the ADC driver IF

- removed UNDEFINED value from device enum
- added pre-defined precision values
- added power[on|off] functions
- some minor doxygen fixes
boards/arduino-due: some cleanup

- added parenthesis for LED macros
- changed mapping for LED_* to LED_RED_*
- removed some old values from periph_conf.h
boards/udoo: cleanup in periph_conf.h

- removed unused defines
Merge pull request #1479 from haukepetersen/fix_sam3x_minorfixes

board - arduino-due: some cleanup
Merge pull request #1469 from haukepetersen/fix_periph_adcif

drivers: some improvements to the ADC driver interface