[navstik] Added a new board
[stm32] fix compilation on F1 boards
[boards] minor fixes for navstik
[ins] properly define INS_VFF_R_GPS

closes #741
[electrical] min bat level check

This allows for detecting when voltage is too low to be a real reading and treats it as invalid.
The GCS shows BAT LOW INVALID.
The ap does not set bat_low or bat_critical if the reading was never valid.
But once a valid voltage is read then low level checking is always done.

closes #745