05core: create top-level user symlinks on first boot

[jlebon]

With this, users can specify an Ignition config like

variant: fcos
version: 1.4.0
storage:
  files:
    - path: /etc/ostree/config.d/01-foobar.conf
      contents:
        inline: |
          [toplevel-links]
          foobar=/var/foobar

and on first boot, a /foobar symlink will be created.

Requires: https://github.com/ostreedev/ostree/pull/2681 Related: https://github.com/coreos/rpm-ostree/issues/337

[jlebon]

With this, users can specify an Ignition config like

You can see a more complete demo of this in https://github.com/ostreedev/ostree/pull/2681.

[bgilbert]

Should we have Butane sugar for the [toplevel-links] section?

[jlebon]

https://github.com/ostreedev/ostree/pull/2681#issuecomment-2138106566