sys/analog_util: Refactor adc_map, fix compilation

Change the API to use int32_t instead of int, to allow for greater
flexibility on 8- and 16-bit platforms. Removed limitation on input
arguments that min < max. Times where it can be useful to have min > max
is when measuring a sensor where a higher measured voltage means a lower
physical value. For example a thermistor can be connected so that the
measured voltage goes down when the temperature goes up.