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