systemd: prefer vmlinuz.efi over vmlinuz

For some kernels in postmarketOS it will be useful to support systemd-booting (via a chainloaded bootloader like u-boot) as well as supporting the existing bootloader which only supports a GZIP compressed kernel image.

To enable this usecase, check to see if a version of the kernel file with the .efi extension exists and prioritise using it for systemd-boot.

This will allow kernel packages to install both the EFI boot stub kernel and old GZIP'd image at the same time.