Make VARIO optional

[haslinghuis]

No description provided.

[github-actions]

Do you want to test this code? You can flash it directly from Betaflight Configurator:

• Simply put #13624 (this pull request number) in the Select commit field of the Configurator firmware flasher tab (you need to Enable expert mode, Show release candidates and Development).

WARNING: It may be unstable. Use only for testing!

[ctzsnooze]

We want to support the user's request to include vario when they additionally include either GPS or BARO, but if they provide neither, then we don't go ahead.
Maybe this should be:

// Remove USE_VARIO if neither BARO nor GPS are defined.
#if defined(USE_VARIO) && !(defined(USE_BARO) || defined(USE_GPS))
#undef USE_VARIO
#endif

[haslinghuis]

Sorted.

[ctzsnooze]

This should go into 4.5.1 I think also.
It restores the old behaviour.

[KarateBrot]

This makes the use of the vario more complicated. In master, you can just use the vario feature if you have the needed sensor on board. With this PR you have to compile a version with vario explicitly/manually activated. You don't get it automatically anymore, if the requirements for its use are met.

As far as I can see, it just complicates the use of Betaflight cloud builds without benefits.

What is the goal of this? Excluding it doesn't even save much flash space. Please add a description to PRs so people know what you want.