Sub: Simplify home logic

[Williangalvani]

When automatically set, home is now Always at the surface. (this was already assumed in copter anyway, and waypoints above home are currently invalid).
get_alt_rel() also got updated to use ahrs.get_relative_position_D_home(posD);, which uses the home instead of ekf origin, and already falls back to baro, which made the baro code redundant.

This fixes my issues with running missions with depth measured from the water surface. specially when using a GPS, which has very bad accuracy on vertical position

[peterbarker]

No longer correct

[peterbarker]

When automatically set, home is now Always at the surface. (this was already assumed in copter anyway, and waypoints above home are currently invalid). get_alt_rel() also got updated to use ahrs.get_relative_position_D_home(posD);, which uses the home instead of ekf origin, and already falls back to baro, which made the baro code redundant.

This fixes my issues with running missions with depth measured from the water surface. specially when using a GPS, which has very bad accuracy on vertical position

I think what you're saying is that after this we no longer project the location onto the water surface when you are armed and get home. So if you're scooting along in depth-hold and suddenly get home then it's exactly where your vehicle is, not where you are but at the surface.

[peterbarker]

@Williangalvani this is approved, you can merge it at will :-)