image_types_balena.bbclass: Move image signing code here

[floion]

This fixes the following error:

cp: cannot stat '/work/build/tmp/deploy/images/generic-amd64/balena-image-generic-amd64.balenaos-img.sig': No such file or directory

The signed image symlink is not generated before the flasher needs it so we avoid this dependency issue by defining a new image type for which the build system will automatically create the appropriate symlink.

Change-type: patch

---

Contributor checklist

• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
• Change-type present on at least one commit
• Signed-off-by is present
• The PR complies with the Open Embedded Commit Patch Message Guidelines

Reviewer Guidelines

• When submitting a review, please pick:
  • 'Approve' if this change would be acceptable in the codebase (even if there are minor or cosmetic tweaks that could be improved).
  • 'Request Changes' if this change would not be acceptable in our codebase (e.g. bugs, changes that will make development harder in future, security/performance issues, etc).
  • 'Comment' if you don't feel you have enough information to decide either way (e.g. if you have major questions, or you don't understand the context of the change sufficiently to fully review yourself, but want to make a comment)

[floion]

Hi @klutchell just a heads up that I pushed a change to this PR but that did not make it invalidate the policy-bot. Even more, I also re-requested another review from @alexgg but policy-bot is still marking the PR as approved.

[alexgg]

Still fails - needs some work

[floion]

Still fails - needs some work

It worked now but fails for an unrelated issue: https://github.com/balena-os/meta-balena/actions/runs/12808128684/job/35710334625?pr=3597