Add descend mode as a failsafe.

[AlexisTM]

The land mode supposes we have a GPS causing issues when flying with a local position (such as mocap or vision)
This adds the alternative to use the descend mode.

Solved Problem

When working on local position systems such as a vision localization, the land mode doesn't fit the need when the localization is failing as we want a way to land.

Furthermore, the current Land mode supposes the use of GPS which is not present using vision/flow/mocap, making the drone try to land at (0,0) with a heading of 0°, which is not the expectation.

Fixes #23773
Fixes #21524

Solution

• Add the use of Descend mode for all failsafe actions

Changelog Entry

For release notes:

Feature: Failsafe modes now include the descend mode

Alternatives

There could be an automatic demotion from Land to Descend

Context

Related links, screenshot before/after, video

[sfuhrer]

I think #23580 already fixes your issue. There I changed the state machine logic such that "Land" failsafe action is always triggered, even if the mode requirements for "Land" are not met. It then instantly falls-back to Descend.

[AlexisTM]

Unlike the bug you fixed where you couldn't enter land mode, we actually enter but want the Descent behavior instead.

There are two other problems we encounter:

• The land mode succeeds as we do have a proper local position, but it tries to go to (0,0) instead of landing in position. That is likely a bug.
• Even when the land mode works properly, and we have a valid local position, that local position could be unstable due to vision input, especially when developing said vision software. So we do not want to rely on the position at all for the emergencies.

[AlexisTM]

It is true that if we add the requirement for land mode to have a global localization, your PR may suffice.