Flare: Use positive pitch during flaring. Relative alt: Use laser altitu...

...de to decide if in range, not relative altitude estimatie which could be off.

@thomasgubler Could you review and comment and/or merge?

This is test-flown yesterday and the pitch set point seems to make sense. Note that I set the flare altitude to 3 meters (probably still can do down there).
btw: we want hysteresis here, or force the system to stick to the range finder once it switched to it