Disallow servorail-only powered flight.

Enforce (in presence of power sensing) that a) system is not purely servo rail powered and b) power rail voltage is higher than 4.5V on the main avionics rail.

Using only the servo rail as input depends on the quality of the BEC and over voltage suppression (Zener) - we can't rely on users getting this all right.

Not yet tested, but good for comments / reviews.