[SRU] Correctly map boot entry to OEM setup

[ Impact ]

Historically with Ubiquity-based live ISOs, a boot entry exists specific for OEM configuration. This has become an expectation from an end-user perspective, for installs dedicated to that purpose.

OEM mode is a feature that is now only present in Calamares-based ISOs, with Ubuntu Desktop Installer-based ISOs having progress tracked in bug 2048473. Out of the three Calamares-based ISOs (Ubuntu Unity, Kubuntu, and Lubuntu), only two of those (Kubuntu and Lubuntu) have dedicated installer prompts to boot right into the installer, where Ubuntu Unity simply has a desktop icon on first boot, which is the same as Lubuntu's functionality from 18.10 to 23.10 in this respect.

Lubuntu and Kubuntu do not have an OEM boot entry enabled (which will require a debian-cd merge proposal), and also do not have this logic in the installer prompt.

[ Test Plan ]

 1. Boot a Noble daily ISO or 24.04.1, if already released, for Lubuntu and Kubuntu.
 2. Select the following boot entry: "OEM install (for manufacturers)"
 3. After system boot, Calamares in OEM mode is shown.

[ Where problems could occur ]

When updating Calamares itself in a future update, care must be taken to ensure OEM mode continues to work as designed. An underlying regression in Calamares has the potential to break the functionality of this update.

Additionally, if debian-cd changes in a non-backwards-compatible way for Noble, this could also break the functionality of this update.

[ Other Info ]

Acceptance of these updates into -proposed must be done after the linked debian-cd MP is merged.

This is a release blocker for 24.04.1, not for 24.04 final.

We plan on merging these two source packages next cycle: https://bugs.launchpad.net/ubuntu/+source/kubuntu-installer-prompt/+bug/2063289