firstboot: adjust what systemd.firstboot=no on the kernel cmdline does

firstboot: adjust what systemd.firstboot=no on the kernel cmdline does

So far by setting systemd.firstboot=no simply short-cut the whole tool
and made it exit early. This is against what the docs say though: they
just claim the user isn't asked for questions anymore. Let's change
behaviour so that the code actually matches the docs, or more
specifically: if credentials are passed into firstboot, then honour
them, regardless of the kernel cmdline option.

After all, if we get explicit data passed in we should operate on it,
and then leave systemd.firstboot=no just affect the interactivity.

I think this was actually mostly a bug introduced because the credential
stuff was added after the kernel cmdline option, hence this just catches
up with the new addition.