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.
Seeing this is intended to be AGL should that be part of the parameter name? Otherwise making it part of the first line of the comment might be enough too.