feat: Auto build sysext overlay for standard UKI images

[bencorrado]

Is your feature request related to a problem? Please describe.

When building a UKI image the firmware of devices can regularly limit the total UKI image size causing a malloc issue that prevents the system from booting. On many firmware packages this appears to not directly be linked to system resources where adding more RAM/disk does not fix the issue.

This creates an issue when creating large UKI files that contain a full rootfs as the system will not boot on some systems. Even jus enabling standard over core variant can cause issues for a lot of hardware.

Right now packaging the sysext and adding it to a UKI ISO build is a manual process.

Describe the solution you'd like

We should attempt to load things into sysext instead of into the rootfs. for standard packaging. Prepackaging a standard variant sysext and having the build process know that when there is a VARIANT=standard and BOOTLOADER=systemd-boot it should not build this into the rootfs like the other standard images, but instead at the osbuilder/enki stage layer on the right sysext as an overlay.

Describe alternatives you've considered

Additional context

Recovery and reset do NOT load sysext by default. If the software placed into the sysext is expected to be used here we should alert the user in docs. We should make sure installer runs the sysext as much of the standard variant needs this (might be there, I have not verified)