Add a "bootctl link-auto" verb and a matching io.systemd.BootControl.LinkAuto()
Varlink method that behave exactly like "bootctl link" / Link(), except that
the UKI and extra resources are discovered automatically instead of being
passed in. The following directories are searched, in decreasing priority:
/etc/systemd/uki/, /run/systemd/uki/, /var/lib/systemd/uki/ (where
systemd-sysupdate stages downloaded resources), /usr/local/lib/systemd/uki/
and /usr/lib/systemd/uki/.
- the UKI is taken from kernel.efi, or the best version in kernel.efi.v/
(resolved via vpick, without honouring boot-counting suffixes), from the
highest-priority directory that has one;
- extra resources are picked up from extras.d/, matching *.sysext.raw,
*.confext.raw and *.cred, each either as a plain file or as a versioned
*.v/ directory resolved via vpick, combined across all directories with
higher-priority directories winning on conflicts.
Everything is resolved relative to the pinned root directory fd. Files passed
via --extra= on the command line are linked in addition to the auto-discovered
ones.
Also bind io.systemd.SysUpdate.Notify.OnCompletedUpdate() in the boot control
Varlink server, which simply does the same as LinkAuto(), and hook a socket
into /run/systemd/sysupdate/notify/ via systemd-sysupdate-notify-bootctl.socket
(enabled by default via the preset) so a freshly downloaded kernel is linked
into $BOOT automatically after a sysupdate run.
Among other things this changes tracking of the location of resources
during GC from using the BootEntrySource enum rather than a path, since
we have that and it is more efficient and easier to grok.
The unlink command removes an entry from the ESP including
referenced files that are not referenced in other entries. That is
useful eg to have multiple entries that use the same kernel with
different options.
The cleanup command removes all files that are not referenced by any
entry.
When using --root=/--image= the binaries to install/update will be
picked from the directory/image. Add an option to let the caller
choose.
By default (auto) the image is tried first, and if nothing is found
then the host. The other options allow to strictly try the image
or host and ignore the other.
