Add supply voltage monitor

We seem to have some precision issues. With a supply of 4.00V (As measured with the UT136B multi meter) we get 4.08V. This is quite an offset. Possible reasons:

VDDA is somehow off (unlikely since it is just connected to 3.3V which also measures as 3.30V)
We should run ADCAL first. Not sure if this is done in the HAL or not) This could remove some offset, but 2% offset seems a lot.
I'm too dumb to do the calculation correctly :wink:

gfroerli / firmware

Add supply voltage monitor #73