40ignition-ostree: always save root partition state

[jlebon]

Right now, there's special logic in coreos-growpart so that we skip growpart if the rootfs backing block device or size changed during rootfs reprovisioning (e.g. if one simply changed from xfs to ext4).

Let's make that logic more generic so that it's not actually tied to rootfs reprovisioning. That way, if a user changes the partition size via Ignition (pending https://github.com/coreos/ignition/issues/924), we know not to automatically growpart.

[jlebon]

Not tested yet.

[jlebon]

I think this could still make sense but let's revive it when we see a clearer need for it.