Merge: redhat: create 'systemd-volatile-overlay' addon for UKI

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/931

JIRA: https://issues.redhat.com/browse/RHEL-92882

Upstream Status: RHEL-Only

Create 'systemd.volatile=overlay' UKI addon. This is option allows to overlay the whole root with an overlayfs which is backed up in tmpfs. Which means that any write will be lost on reboot.

This option is especially useful when the root is dm-verity protected, which means it is mounted as RO, and the workload type running on the system doesn't rely on storing data, such as running containers.

Note that systemd.volatile=state doesn't really work on Fedora/RHEL, as it just mounts tmpfs on /var but root stays RW (should be RO according with docs), and systemd.volatile=etc puts a tmpfs on top of root, resetting the system (and most importantly /etc) completely which then makes the system unusable.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

Approved-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Approved-by: Jan Stancek <jstancek@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: CKI GitLab Kmaint Pipeline Bot <26919896-cki-kmaint-pipeline-bot@users.noreply.gitlab.com>

Author: CKI KWF Bot

redhat/uki_addons/common/systemd-volatile-overlay.addon

@@ -0,0 +1 @@
+systemd.volatile=overlay