Support multiple kernels for one boot environment (systemd-boot)

[rpthms]

Is it possible to make the systemd-boot plugin support multiple kernels for one boot environment? Currently it just clones the org.zectl-<BE>.conf to create a new loader entry. But that file can only contain one kernel. Other kernels can also be installed simultaneously on Arch, like the linux-lts kernel. Maybe we could name other kernel conf files as org.zectl-<BE>@lts.conf or something like that and have zectl clone every conf file with <BE> in it.