Unblock baro connection check when external depth is available on mode switch to ALT_HOLD

[ahoEIVA]

Feature Request

I would like to switch to ALT_HOLD mode when depth sensor is not connected to the board.
Depth/Altitude is provided from an external source, and EK3_SRC1 parameters are configured to use GPS or ExternalNav.
(

ardupilot/ArduSub/mode.cpp

Line 100 in 7db137f

!sub.control_check_barometer()) { // maybe use ekf_alt_ok() instead?

)

Describe the solution you'd like
There is already a suggestion in the code to implement altitude ok flag, I assume this could be true in above described scenario, or if baro is connected.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[x] Submarine