check composefs compat when rebasing

[cgwalters]

Our default stance for bootc is to enable composefs, which requires sysroot.readonly=true, which in turn imposes other requirements/tweaks like ensuring the rootfs is mounted read-write in the initramfs with rw, etc.

It's a trap today if one is bootc switching from a system without this setup to a bootc base image that is, because we'll barf in the initramfs.

Short term, I think we can check at least before the update (and reboot) for this situation. Hmm, something we clearly should have more of but don't is static checks between the phases:

• download new rootfs
• <static checks>
• queue bootloader entry

rpm-ostree at least does a bwrap invocation of true in the new root, which has verified some things in the past.

And the composefs one is pretty easy to check; look in the new root for /usr/lib/ostree/prepare-root.conf and check if it's enabled in the new one, but the current root doesn't have sysroot.readonly.

[cgwalters]

Also one thing people are starting to hit is ostreedev/ostree#3198 - we could definitely detect the case where one is using dynamic grub configs and the target is using composefs and at least error out for now.