bootctl: update/list/remove all instances of systemd-boot in /EFI/BOOT

bootctl: update/list/remove all instances of systemd-boot in /EFI/BOOT

systemd-boot might be installed in /EFI/BOOT under more names than
just /EFI/BOOT/BOOTX64.efi. The prime example is shim which loads
its second stage binary from /EFI/BOOT/grubx64.efi. To accomodate
use cases where systemd-boot is installed as /EFI/BOOT/grubx64.efi,
let's always check the entire /EFI/BOOT directory for binaries that
identify as systemd-boot and list/update/remove those as well.

Let's keep this somewhat generic though and not install ourselves as
grubx64.efi since that would mean having to check for shim which is
a can of worms we probably don't want to open.