ekf2: Reinstate saving of mag declination for use next start

[priseborough]

Fixes #10023

The capability and parameter selected default behaviour to save the magnetic declination for the last location had been removed unintentionally by previous changes. This PR reinstates the behaviour.

Requires PX4/PX4-ECL#486 to be merged first.

Reported here

Test data / coverage

1. Start QGC and connect autopilot via USB
2. Load parameters and set EKF2_MAG_DECL to 0
3. Repower board and wait for GPS checks to pass.
4. Reload parameters in QGC and verify EKF2_MAG_DECL has been updated.
5. Repower board with GPS disconnected and and check EKF2_MAG_DECL value has been saved. See screen shot below

[dagar]

Any idea when (or why) this disappeared?

[dagar]

You can actually still use _params.mag_declination_deg directly. All you need to do is decide when to commit the parameter ekf2 side. The internal value of _mag_declination_deg is a reference to _params.mag_declination_deg.

[priseborough]

No idea when or why the functionality was lost.

I don't understand your second comment about using _params.mag_declination_deg directly.

[dagar]

It's not critical, but you wouldn't have to actually copy the declination back to ekf2_main, and ecl/EKF side you wouldn't need _mag_declination_to_save_deg.

In ekf2_main _mag_declination_deg is a ParamExtFloat for EKF2_MAG_DECL. The internal value of _mag_declination_deg is _params->mag_declination_deg.

[dagar]

We should also set EKF2_MAG_DECL volatile (parameters automatically changed vehicle side).

[priseborough]

So you are saying we should modify ecl to write the value retrieved from the geo library directly to the parameter value rather than provide it via an accessor function?

I will give you a call to clarify the use of the volatile attribute.

[LorenzMeier]

Let's merge as-is and follow up to this PR if needed.

[LorenzMeier]

@priseborough ECL updated.