Test booted the resulting ISO image successfully.
Before:
```
$ xorriso -indev ../bootonly.iso -report_el_torito plain -report_system_area plain
xorriso 1.4.8 : RockRidge filesystem manipulator, libburnia project.
[...]
El Torito catalog : 19 1
El Torito images : N Pltf B Emul Ld_seg Hdpt Ldsiz LBA
El Torito boot img : 1 BIOS y none 0x0000 0x00 4 20
El Torito boot img : 2 BIOS y none 0x0000 0x00 1600 24
[...]
```
After:
$ xorriso -indev ~/freebsd/bootonly.iso -report_el_torito plain -report_system_area plain
xorriso 1.4.8 : RockRidge filesystem manipulator, libburnia project.
[...]
El Torito catalog : 19 1
El Torito images : N Pltf B Emul Ld_seg Hdpt Ldsiz LBA
El Torito boot img : 1 BIOS y none 0x0000 0x00 4 20
El Torito boot img : 2 UEFI y none 0x0000 0x00 1600 24
[...]
```