Plane: RTH Stepped Altitude

Would love to see altitude steps (or just one step) added to RTH mode in Arduplane. I think Arducopter already has this.

e.g.

Alt Hold RTH when distance from home is less than 500m = 80m
Alt Hold RTH when distance from home is greater than 500m = 150m

With the ability to set the distance of the step and both altitudes.

Start by locating ArduPlane's RTH mode implementation and the existing ArduCopter altitude-step behavior mentioned in the issue. Compare how distance from home and return altitude are handled, then determine the parameters and tests needed for a configurable distance threshold and two RTH altitudes. Done means the requested stepped-altitude behavior is implemented and verified without regressing existing RTH behavior.

Written by the indexing model from the issue text.