make sure the arch templates (s390 excluded) copy the runtime image in to the /images/ dir, and make sure it gets grafted/copied into the boot.iso.
TreeBuilder uses templates full of commands (like ramdisk.ltmpl) to create the output tree and boot images. There are 4 arch-specific templates, plus a bonus EFI template which can handle EFI image creation for any arch that implements EFI.