Port DigitalOcean OEM away from the base Ignition

[pothos]

Current situation

The DigitalOceam OEM setup uses a base Ignition file that gets merged with the user-passed Ignition config.
Currently the only action is to enable coreos-metadata-sshkeys@.service.

Impact

The setup writes state to the rootfs that is not updated.

Ideal future situation

We don't use the base/default Ignition approach but have a unit in the generic image that enables/runs/depends-on coreos-metadata-sshkeys@.service with a condition for KernelCommandLine=coreos.oem.id=digitalocean or KernelCommandLine=flatcar.oem.id=digitalocean.

Additional information

We could also create a sysext image but that does not make much sense, we already ship KernelCommandLine= OEM units in the base image and the initrd.

A migration action is not really needed for this limited case. But if, we could do it live in flatcar-postinst based on the current and next version being ≥ the major version that introduces the change.

[tormath1]

Closed by flatcar/scripts#1197