[Spike] Make UKI systemd sysext solution more robust

[jimmykarily]

Instead of XBOOTLDR, we decided to go further in the path of systemd sysext}.

This ticket is to investigate the limit of this solution and find ways to support extensions as big as possible. We have reasons to believe that the system extension size is limited to a very low size (~500Mb) and we want to see if it's possible to load bigger ones.