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).
The problem I see with this is that this triggers if you fly over hill/obstacle in your approach. I used `rel_alt_estimated` because I thought that it is exact enough for the switch. If `range_finder_use_relative_alt` is large enough the very exact point for switching does not matter.