[arch-releng] Building LTS iso

I am trying to use archiso scripts to build an LTS kernel version of iso but not having much luck. Have of course modified build.sh (vmlinuz etc..). At this stage I have only added in linux-lts as an extra package. Have a feeling I am missing something else wondered if anyone has tried lts version or know what might be wrong? Iso boots but cannot find root.fs.sfs (even though it is there)