upgrade doesn't support initramfs <-> non-initramfs switch

[mcorb]

Commit 4233b1d and #442 added backend support to transition to/from initramfs, but ostree admin upgrade doesn't support --karg-none

As a result, the old init= gets silently passed to the new boot args, and the system fails to boot when you go either way.

It looks like a relevant change was made in 652d9dd but we couldn't figure out how to make it work with the upgrade command.

[cgwalters]

Hmm. I think we could pretty easily add support for something like having default kernel arguments as part of the commit or so?

Bigger picture though, this seems similar to: #1228

In that I think a lot of people will have these types of "checkpoints" that are best handled by a higher level tool. I know the transition to "libostree" here is a bit rocky, and people are using ostree admin upgrade today in production. We're not going to break it, but it's tricky to have it grow a lot of features as it starts to intersect a lot with how the distribution is made.