buildlive: Cleanup

A lot of variables are now self attributes to avoid passing them as
arguments.

Live image data is in self.data instead of self.data["live"].

The "application" and "publisher" entries are now optional.

Require isolinux.bin instead of isolinux-debug.bin, the base system
now has syslinux-isolinux installed.
Fail if one or more architectures are not supported

Make the build process more robust.