Support base and user configs

[bgilbert]

We will need to support:

• Platform-specific base configs. (Also, default configs, if Drop support for platform default configs ignition#668 is not implemented.) Unlike in CL, these should probably all be kept in the initramfs image.
• User-supplied config.ign, perhaps in the /boot partition.

In CL these are handled by ignition-setup.service, which is currently disabled here because the original version relies on the OEM partition.

[bgilbert]

Unlike Container Linux, in which platform agents are kept in a separate partition outside the base OS image, Fedora CoreOS will include platform agents directly in the base image. So, in general, it doesn't make as much sense to enable agents using Ignition. Instead, the service units can also be shipped in the base image and conditionalized with a ConditionKernelCommandLine or similar.

GCE OS Login enablement may be an exception, since we have to override config files in /etc but give the user the option of overriding them back.

[yrobla]

I think that being able to ship user-supplied config.ign is something needed. For the baremetal use case is specially useful, as there are no other ways to parameterize. In case of PXE boot, people could add kernel parameters with the coreos.config_url and it will work. But in case of using virtual media, modifying kernel parameters is not an option, so having the ability to embed some config.ign in the image for modifications like inject ssh key, start some services, etc... is very useful.

[cgwalters]

User-supplied config.ign, perhaps in the /boot partition.

👍 - seems simple to implement and solves the problem.

[dustymabe]

@yrobla would you like to try to implement this and open a PR?

[yrobla]

Sure, happy to work on that!