Minimum altitude in auto

@sjwilks @julianoes @DrTon @thomasgubler This change enforce the minimum altitude in loiter if no mission is loaded. Please review carefully.

This is untested. @sjwilks A flight test would be cool, but test carefully.
We need it for any situation where the horizontal part of the flight path is not well defined. If the user wants to loiter low we should allow him to (e.g. he could start off a hill and fly into a valley).