Add docs for systemd-sysupdate

[pothos]

Current situation

systemd-sysupdate is an experimental new part of systemd for partition and extension updates. So far it landed in Flatcar Alpha but there are only upstream docs.

Impact

This makes it hard to Flatcar users to quickly see how to use this.

Ideal future situation

A part of the Flatcar sysext docs should show how to use systemd-sysupdate with Ignition examples.

Implementation options

The current-symlink setting should be used and because we don't want to clutter /etc/extensions/ with the version-suffixed images, we better store all versioned images of one extension in its folder, and set up a /etc/extensions/NAME.raw symlink to the current version in that folder.

Additional information

[tormath1]

Documentation example: https://github.com/flatcar/sysext-bakery#consuming-the-published-images

[pothos]

First docs are in: https://www.flatcar.org/docs/latest/provisioning/sysext/#updating-custom-sysext-images
I think we should extend this a bit to use sysupdate components, otherwise the version can't be controlled well if two extensions should be updated.

[tormath1]

@pothos do you think it could be something interesting for the upstream to have a systemd-sysupdate@.{service,timer} ?

[pothos]

Interesting idea. As alternative one could also work with ExecStartPre= drop-ins for the component updates and a ExecStartPost= drop-in to issue systemctl restart systemd-sysext systemd-confext.

[tormath1]

Using ExecStartPre= drop-ins for each components won't avoid the ExecStart= to be executed from the systemd-sysupdate.service 🤔

[pothos]

Yes, I've added a noop.conf file now to silence the error message.

[pothos]

We should test the new update daemon service and see if that works for us.

[pothos]

Yes, I've added a noop.conf file now to silence the error message.

Another trick we could do is to symlink the service and timer units with a new name and use a drop-in to overwrite ExecStart=. But I kind of like to have only one timer/service to trigger for everything, and no custom unit names that people first have to learn about.

[pothos]

PR flatcar-archive/flatcar-docs#329