ignition: systemd: fix systemd 256 usage

[ader1990]

Systemd 256 mounts /usr as read-only, but Ignition setup requires to have some files written depending on the live image or the ignition contents.

The are multiple options we can consider:

• this option, where we make /usr read-write and do the necessary changes
• another option would be to setup symlinks which redirect to /tmp or a writable file we can write
• another option would be to have the ignition binaries pre-baked by build_image

Testing done

[Describe the testing you have done before submitting this PR. Please include both the commands you issued as well as the output you got.]

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[ader1990]

Just found out from @tormath1 (thanks!) that Fedora CoreOS had the same issue and implemented similar fixes: coreos/fedora-coreos-config@44d0ff8 coreos/ignition#1891