Add explicit power output to digital I/O port on BB3 chassis

[fietser28]

To power small auxiliary extension that are connected to the BB3 it is usefull to have an additional port or make explicit which port power can be drawn from. These hardware extensions (sensors, switches) are very powerful in combination with the micropython capabilities of the BB3.

(As a workaround) it is now possible to use Dout2 (making it high) to power very small circuits.

Power can be low: e.q. 3.3V/25mA and might nog even require a hardware change, just labeling and documentation and maybe an explicit software feature instead of 'output high'.

A bigger change would be to add an additional power only pin, but this is not backwards compatible with current systems.

[prasimix]

We could also add switch that could reroute VBUS controlled by IC11 (TPS2051) to Dout2 terminal.