Implement systemd-boot boot assessment

[jimmykarily]

systemd-boot has a was to perform boot assessment and fallback to other entries if booting fails. It is described in detail here and here. It's not very complicated and only requires us to name the conf/efi files in a certain way and also make sure we order entries properly (so that the right one is picked as a fallback).

Note: Originally investigated while documenting how Kairos does boot assessment,