check composefs compat when rebasing

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.

Inizia dal flusso di bootc switch e segui le fasi dal download del nuovo rootfs fino all’accodamento della voce del bootloader. Esamina come la nuova root viene resa disponibile per i controlli e confronta il suo /usr/lib/ostree/prepare-root.conf con l’impostazione sysroot.readonly della root corrente. Il lavoro è completo quando i rebase composefs incompatibili vengono rilevati prima che l’aggiornamento o il riavvio procedano.