BTDT using MSL not ASL for height measurement

[RWOne]

Example, on mun or minmus, using BTDT on a rover, on ground but above 2k msl, unit status will state too high.

Needs to use AGL, not MSL for measurement of altitude so unit will function on ground regardless of MSL height.

[tofof]

Confirmed, this prevents the BTDT from being used in a ton of situations including, for example, the bulk of the Mun.