Beta firmware - 0.4.2

Implemented a coarse approximation for the -12V rail monitor

IMPORTANT! Support for rev 5 has been (temporarily?) dropped! If you need something that kinda works on the older PCB, stick to v0.3.0!

My brain started hurting real bad trying to understand how to work with the voltage divider used to monitor this negative voltage rail. At some point, I decided to run the circuit into a SPICE simulator, only to find out the voltage clamp clamps too high and the ADC gets saturated very early on. Also, in the active ADC range, the behavior is almost perfectly linear, so I decided to apply an approximated line instead.

According to my tired calculations, the realistic range is -34V up to -8.6V. Anything above that is harshly truncated to 0V. (I guess I could replace it with a more graphically appealing LOW or THRS, but there's always time for that...)

If anyone smarter than me is able to comprehend these EE shenanigans better than me, please chime in and fix my ugly hacks.

Happy PicoPOSTing!