I have done some reading up and made changes to my build.sh to follow
arch wiki guide
So I now use
mkarchiso -r "mkinitcpio -c ${script_path}/mkinitcpio.conf -k
/boot/vmlinuz-linux-lts -g ${_dst_boot}/${arch}/archiso.img" run
Which is the correct way :)
I only noticed --basedir stuff today when I updated mkinitcpio
/me slaps wrist....
MrG