fix(bootloader): enable graceful to workaround fw bug

Changed files
+1
systems
+1
systems/koumakan/configuration.nix
···
boot.loader.systemd-boot = {
enable = true;
netbootxyz.enable = true;
};
···
boot.loader.systemd-boot = {
enable = true;
+
graceful = true;
netbootxyz.enable = true;
};