hostapp: Move supervisor update logic from balenahup

[floion]

Right now the hostOS update is done through a call to the balenahup proxy action. The code is https://github.com/balena-os/balenahup/blob/master/upgrade-2.x.sh

We are moving away from this OS update path over to having the supervisor handle OS updates. As such, we need to move some of the steps from upgrade-2.x.sh over to the hostapp-update script.

For a time, until the supervisor managed OS updates are implemented, we will still be using OS updates through the balenahup proxy action with the changes in this commit which should still work as before.

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)

[alexgg]

I'd probably like to understand what is the rational for the hostOS having to call the API for this and whether whatever is triggering the hostOS update should do this instead.

[floion]

Replied at https://balena.zulipchat.com/#narrow/channel/346007-balena-io.2FbalenaCloud/topic/Roadmap.20to.20tri-app/near/486479671