Copter: RTL returns at incorrect altitude if EKF origin alt different from Home alt

[rmackay9]

During Copter-4.0.2 testing I noticed that my vehicle was descending a couple of meters in the first stage of RTL. The vehicle should never descend when it enters RTL, it should always return at the current altitude or climb to the greater of RTL_ALT and (current altitude + RTL_CLIMB_MIN).

This can be reproduced in SITL by doing the following:

• wait until the, "EK2 IMU0 origin set" message is displayed
• param set GND_ALT_OFFSET -3
• param set RTL_ALT 500
• loiter
• arm throttle
• increase throttle and forward pitch to fly the vehicle to at least 10m above and away from home
• RTL

The vehicle will descend 3m and return home.

Here is a graph of CTUN.Alt from the above test.

[rmackay9]

For accountability this is probably the PR that caused the issue by chaning current_loc.alt to be an altitude above origin instead of an altitude above home. #10587

[rmackay9]

This is fixed in master and the fix will also go out with Copter-4.0.2-rc4