uc20 grade dangerous images don't seed properly with devmode snaps in them

[Ian Johnson]

Dangerous model images currently cannot seed at all devmode snaps, failing during first boot of run mode thusly:

[ 6.935996] snapd[900]: stateengine.go:150: state ensure error: devicemgr:
[ 6.937319] snapd[900]: - cannot seed: snap "hello-world" requires devmode or confinement override

This is because support for `snap prepare-image` to write anything into options.yaml (or somewhere else?) about needing to install the snaps using devmode is missing.

[Maciej Borzecki (maciek-borzecki)]

Fix proposed by Ian: https://github.com/snapcore/snapd/pull/9463

[Ian Johnson (anonymouse67)]

Maciej, that PR is orthogonal, that simply prevents folks from creating non-dangerous grade images with devmode snaps in them, I have not yet proposed a fix for _dangerous_ images seeding properly with devmode snaps, though I do have a working branch locally which fixes this. It's a bit of a pain to write tests for that though so I have not proposed it yet, but hope to do so soon.

[Ian Johnson (anonymouse67)]

RFC PR here: https://github.com/snapcore/snapd/pull/9745

[Ian Johnson (anonymouse67)]

That PR is in the wrong direction, but we will sync and figure out the right direction next week, so setting this to in progress

[Ian Johnson (anonymouse67)]

This was implemented in https://github.com/snapcore/snapd/pull/9798