Calibration: calculate the z-accel offset when leveling

This calculates the offset that will make the z-accel average
gravity after leveling.
Calibration: connect the bias leveling method to a button

The user can either click a button to calculate the board rotation with
or without zeroing the z-accelerometer.
Attitude: move the zero gyros option check box

It was grouped with the leveling button (historically) but is
an independent option so doesn't really belong there.