Functionality to upload height data manually to the flight controller - Terrain Following

#### Issue details
When we want to use the Terrain Following functionality using Mission Planner, we must meet a number of conditions.
1.- Activate the functionality Terrain Following -> TERRAIN_ENABLE 1
2.- Download the height maps and images of the area where we want to fly
3.- We must check that in Mission Planner we have the height data with the inscriptions SRTM or GeoTiff (in case of using another manual data source).
4.- We create the mission and upload it to the flight controller.
5.- We have to wait for the height data to be uploaded to the flight controller and we check it using the variable "ter_pend", or height data pending upload. For this to happen we must have satellite coverage, 3DFix.
6.- We can execute the mission.

I see a "glitch" at point 5. The waiting time for the data to be uploaded in case of very large missions and we do not have to spend bandwidth on telemetry.

Would it be possible to implement an option so that executing it manually, when we have fulfilled all the requirements, up to point 4, we can upload the necessary height data to the flight controller without having to have obligatory satellite coverage?

This would allow us to plan the missions quietly at home and not worry about anything else in the flight zone other than flying.

Thank you

Greetings

#### Platform

[X] Copter
[X] Plane

#### Hardware type
Pixhawk 2.4.5, 2.5.6

The issue does not name any files, tests, or entry points. Start by locating Mission Planner's Terrain Following height-data upload flow and determine how manual upload could bypass the satellite-coverage requirement; done means required height data can be uploaded before flight without telemetry bandwidth or satellite coverage.

Written by the indexing model from the issue text.