prepare-root: add kernel cmdline ostree.prepare-root.readonly

[ruihe774]

As described in #3315.

[openshift-ci]

Hi @ruihe774. Thanks for your PR.

I'm waiting for a ostreedev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[cgwalters]

I'm not opposed to this, but I'd like to try a bit harder to reach another design where we can mount the root writable in the initramfs. Let's debate that in the issue #3315 I think to keep it in once place.