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.
I don't think it should be in the Base class. Firstly it has something to do with Mission only and nothing with the other modes, secondly the param is defined in mission_params.c and not in mission_mode_params.c.

Also, I think this will yield multiple parameters like RTL_TAKEOFF_ALT, LOI_TAKEOFF_ALT, MIS_TAKEOFF_ALT which all don't make sense.