ipxe-boot-server: The generated ISO fails to boot in the mupuf infra

It seems like the iPXE-generated ISO isn't working at my farm, when the approach taken by netboot2container worked:

# Syslinux
cp $(SYSLINUX_DIR)/bios/core/isolinux.bin $(BUILD_DIR)/
cp $(SYSLINUX_DIR)/bios/com32/elflink/ldlinux/ldlinux.c32 $(BUILD_DIR)/
cp $(SYSLINUX_DIR)/efi32/efi/syslinux.efi $(BUILD_DIR)/
cp $(PWD)/config/isolinux.cfg $(BUILD_DIR)/

# iPXE
cp $(IPXE_BIN) $(BUILD_DIR)/
cp $(PWD)/config/default.ipxe $(BUILD_DIR)/

# Generate the bootable iso
xorriso -as mkisofs -graft-points -b /isolinux.bin -no-emul-boot -boot-info-table -boot-load-size 4 -c /boot.cat -isohybrid-mbr $(SYSLINUX_DIR)/bios/mbr/isohdpfx.bin -eltorito-alt-boot -e syslinux.efi -no-emul-boot -isohybrid-gpt-basdat -V NETBOOT2CONTAINER -o $(OUTPUT_ISO) -r $(BUILD_DIR)/ --sort-weight 0 /

The main difference in the generated output is the lack of syslinux.efi in the iPXE-generated image.

This is not an urgent task (I re-packed the ISO myself), but it should also not be super hard to fix :)