Copter: Make ALT HOLD mode an option

[muramura]

Many microdrones weighing less than 100 g are flying in Japan. Microdrones are flying with ArduPilot. The base of this micro drone is without GPS and barometric sensor. For this drone, a barometric sensor is not essential. I make ALT HOLD mode an option.

ARDUWHOOP https://youtu.be/eG3bwLJz_a8?si=Joaq2iIjQX6ZKcVm

[muramura]

@peterbarker san. Added ALTHOLD to build_options.py.

[peterbarker]

Ping @muramura .

This needs a rebase.

Henry's pointed out build_options.py would need a change.

I pointed out the class definition shouldn't be present.

Did you still want to pursue this?

[peterbarker]

No response from author. I don't mind the change, but this is now conflicting.

Please re-open after resolving issues if you want to progress.