Post image hooks

[mdhorn]

Fixes Issue: #531

Changes proposed in this pull request:

• Add post-image YAML hook
• Make hooks indeterminate progress
• ensure we read the partition table when mounting loopback images

[mdhorn]

@gtkramer Please take a look at this when you get a moment.

[karthikprabhuvinod]

@mdhorn should we add this something to the baseline.yaml to make sure that this always run as part of make check-image and fails if someone accidentally removes the code?

[mdhorn]

Thinking of it now, with this defined, I see that we dont have a post-image defined, it assumes it to be empty. So dont really see a way of automating it in tests.

I did update the baseline check target to use a post-image hook to compress the image and updated the check in the make target.

Maybe also good to squash the commits. Upto you.

In this case, I wanted to keep the three commits. storage: ensure we read the partition table is sort of related, but something new we needed to as due to recent OS update. controller: Make hooks indeterminate is a separate change I've been meaning to do for a while, but it became even more noticeable when we had a single, long-running post-image hook.

apart from a rebase, LGTM otherwise.

[karthikprabhuvinod]

@mdhorn Nice LGTM :)