In reply to @nikstur:matrix.org
You can just put a boot loader in $esp/EFI/BOOT/BOOTX64.EFI and then you have a working disk image

In reply to @elvishjerricco:matrix.org
yes but this is kind of being a bad EFI citizen.

In reply to @elvishjerricco:matrix.org
you should really have your own os directory in the ESP and create a boot entry efi variable for the firmware to see

$ systemd-repart --empty=create --size=1G --image=image.img --definitions=./defs
Failed to set up loopback device for /home/arian/image.img: Permission denied

[arian@utm:~]$ systemd-repart --empty=create --size=1G --definitions=./defs ./image.img
/home/arian/defs/esp.conf:3: CopyFiles= path is not absolute, ignoring: ./files
Sized './image.img' to 1.0G.
TYPE LABEL UUID                                 FILE     NODE                SIZE PADDING
esp  esp   b43a24f9-410d-4604-8d83-3aecf33a7313 esp.conf ./image.img1   → 1022.9M    → 0B
                                                                      Σ = 1022.9M  Σ = 0B

nice!

[arian@utm:~]$ systemd-repart --empty=create --size=1G --definitions=./defs ./image.img
Sized './image.img' to 1.0G.
TYPE LABEL UUID                                 FILE     NODE                SIZE PADDING
esp  esp   b43a24f9-410d-4604-8d83-3aecf33a7313 esp.conf ./image.img1   → 1022.9M    → 0B
                                                                      Σ = 1022.9M  Σ = 0B

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 └─ esp.conf                                                                                                                  

Applying changes.
Wiped block device.
Discarded entire block device.
Successfully wiped file system signatures from future partition 0.
Formatting future partition 0.
mkfs.fat 4.2 (2021-01-31)
Could not find mcopy binary.

now complaining about mcopy