Bug#514237: debian-cd: Support non-i386 mirrors, and support D-I modules in non-main

[Frans Pop <elendil@planet.nl>]

On Wednesday 11 February 2009, Steve McIntyre wrote:
> In fact, I'd say the simplest answer would be to pick the first named
> arch. If we're doing source-only then don't run update_tasks at
> all. How does that sound?

Bad IMO. It means:
- m-a image may be different when built with "i386 amd64" than when built
  with "amd64 i386"; this is even more true for "i386 powerpc" versus
  "powerpc i386"; IMO the order in which arches are listed should not
  change the resulting image
- source only set will have packages in in a completely different order
  from the corresponding binary set