Level calibration

This adds an additional button that will compute the board rotation to level the airframe AND calculate
an offset to the z-gyro to result in it average gravity. This is useful for multirotors where small errors in
that (which I and other reproducibly get after 6-point calibration) accumulate in the vertical velocity
estimate and result in an attitude shift when engaging the mode.

The z-gyro offset _could_ be calculated and added into the bias terms, but this would destroy the 6-pt
calibration which is tedious to redo. Instead I went ahead and created an addition offset field which is
applied in the firmware _after_ rotating the sensors into the airframe reference.

This button won't necessarily be useful for everyone but is a nice option to have for doing autonomous
flight on multirotors for sure.