Initial import of low-level ADC driver interface

Proposal of a low-level ADC driver interface.

Very simple ADC interface for single conversions that have to manually triggered. Most platforms are capable of many more fancy functions, but for simple usage this interface should suffice...

The mappings to pins and hardware conversion units is statically defined for each board in the 'periph_conf.h'.

The two helper functions are very useful, as most times ADC values are representing physical values that have completely different ranges, e.g. 10-bit ADC values from 0 to 1023 could be mapped to temperature values from -40.0°C to 120.0°C.