Make flatcar-update provide OEM payloads

[pothos]

Current situation

The flatcar-update tool supports downloading the generic update payload or specifying it as file in the CLI parameter.
Nebraska added support for additional payloads that are added after the first one, and are planned to be used for OEM update payloads (as sysext images).

Impact

For developing the update-engine changes one would currently need a local Nebraska instance but if flatcar-update supports passing the OEM payloads, it could be easier to iterate over the update-engine changes.
When released, users will expect the flatcar-update tool to work as before but now it also must cover OEM updates.

Ideal future situation

The flatcar-update tool should get OEM-payload-aware, too, and send the OEM entries in its Omaha response. It should download all OEM files or accept them to be passed as CLI parameters like the generic payload. A downgrade of a freshly installed or a completely migrated machine to a pre-sysext release for that OEM is not supported because the OEM contents will be missing. Ideally, while the sysext OEM availability is tied to a minimal Flatcar version where the basic machinery is present, the availability of OEM sysext images can't be assumed to give us time for the OEMs to get ported one by one; also new ones may be added later. When an OEM sysext update is not published, the system should try to use the old one by forcing its load, and put a warning on the login message (→tbd in the service that activates the OEM sysext images).

Implementation options

Additional information

[pothos]

We can restrict the OEM ID to the current one or the one passed as CLI flag

[pothos]

PR flatcar/init#101