overlay sys-apps/systemd: Sync with Gentoo, bump to v255.4 and add patches for mutable overlays functionality

[krnowak]

Synced with Gentoo, with unnecessary stuff being dropped. This also bumps systemd version to v255.4.

Added the patch implementing the mutable overlays in systemd-sysext - the patch is a merged patch of all the changes I have made and are already in upstream systemd. Not merging the patch would result in having 20 or so extra patches, so it'd be a hindrance. The merged patch implements the minimal mutability and a command-line flag, env var support, ephemeral mutability and some fixes. Changes with documentation and tests are not backported.

CI: #1753

• image diff
• changelog

[ader1990]

I think we can also update to the gentoo upstream of 255.4 without any files content changes other than a mv on the ebuild file and syncing the upstream gentoo sdk_container/src/third_party/coreos-overlay/sys-apps/systemd/Manifest.

[github-actions]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/8803723772

[krnowak]

@ader1990: Could you give me a pointer about reasons for adding sys-apps/kbd to dependencies? I'll update this comment then.

[ader1990]

sys-apps/kbd contains the loadkeys binary, required by the dracut module https://github.com/dracutdevs/dracut/blob/master/modules.d/10i18n/module-setup.sh, which in turn is required by the dracut module of systemd - vconsole setup https://github.com/dracutdevs/dracut/blob/master/modules.d/00systemd/module-setup.sh#L45 and https://github.com/dracutdevs/dracut/blob/master/modules.d/00systemd/module-setup.sh#L106

[ader1990]

More info here: fcfbf18

[ader1990]

and here: flatcar/bootengine#87

[ader1990]

Without this commit, fcfbf18, the initrd size will grow by 3MB.

[krnowak]

CI passed.