Cross compiling and Bootstrapping stories

Use cases for cross-compiling of and in Debian-based systems. Including bootstrapping new flavours and architectures from scratch.

Known use cases to satisfy are:
 1) ALIP/AEL images, which must be cross-buildable, and bootstrappable as far as an X desktop.

 2) Cross-autobuilder/continuous integration. Needs buildd integration - separate cross-dependency satisfaction from sequencing.

 3) Toolchain release testing.
  Want to test new toolchain releases by building a lot of stuff and running an image built from it. This is much faster if cross-building, and more resource is available.
  Issues: Their releases are tarballs, not packages, so some munging is needed - either adding packaging info, or making 'fakedebs' with links and equivs.

 4) OEM integrators basing images on Ubuntu.
  They modify sources and rebuild the modified packages often. They only build against the standard Ubuntu arm archive (and their own packages), so no bootstrapping is needed. They do not have sufficient arm resource for native building, so cross-building of a selected set of packages is important. They do need to be able to build from a local repo of modified sources, and upload to a local repo, using apt priorities to ensure correct sources are used.

Important questions:
1) dpkg-cross vs multiarch cross-building. when/how can we switch? Can we mix?