Enable VREF for battery level monitoring

[cracked-machine]

LED should flash when low battery threshold is reached.

• The internal voltage reference (V REFINT ) provides a stable (bandgap) voltage output for the
  ADC and comparators. V REFINT is internally connected to the ADC V IN[17] input channel.

• VREFEN bit must be set to enable the conversion of ADC V IN[17] (V REFINT).

• During the manufacturing process, the calibration data of the temperature sensor and the
  internal voltage reference are stored in the system memory area. The user application can
  then read them and use them to improve the accuracy of the temperature sensor or the
  internal reference.