[subsystems/electrical] fix conversion of vsupply to decivolts
* Should now work correctly with something like:
  <define name="VoltageOfAdc(adc)" value ="(VOLTAGE_ADC_SCALE * adc + VOLTAGE_OFFSET)"/> with VOLTAGE_OFFSET in Volts
* Also fix VoltageOfAdc to DefaultVoltageOfAdc in board header for tiny_2.0