Every .iso was assumed to be bootable since the very beginning[*],
and isoboot images were deemed to be x86 isolinux ones; this didn't
change with basic ppc/armh support as I never ran into hardware
that would _boot_ those ISOs, not only run the code, and it was
only e2k isodata project that finally forced this refactoring.

It's still not perfect: pack and syslinux features still end up
somewhat interwoven, and too much places care for architecture
the image is being built for (instead of archdep features tossing
their appropriate bits and pieces in).

Should help:
- any-arch regarding isodata images;
- {x86,aarch64}/efi by decoupling isoboot and isolinux;
- ppc{,64} as introducing yaboot support will be easier now;
- mipsel{,64} too, hopefully.

* I knew of school addon images baked with mkimage-profiles-desktop
  but postponed and then neglected the whole problem for years...