Comment 4 for bug 471148

Here's the implementation plan, as agreed with Jono:

1. Add a bool column "restricted" to ProcessorFamily, which will be "true" for "arm". This will tell SourcePackagePublishingHistory.createMissingBuilds() that it should not create builds for restricted architectures.
2. The existing table ArchiveArch associates archives and processorfamilies. If a row exists for a certain archive and arm architecture, then createMissingBuilds() will treat it as an override and ignore the "restricted" flag.
3. Amend the ppa +admin form to allow an admin to specify which restricted ProcessorFamilies the PPA is allowed to use (which will be saved in the ArchiveArch table)

createMissingBuilds() calls determineArchitecturesToBuild() in the buildmaster module. It already uses P-A-S to override the architectures selected - it can do something similar with ArchiveArch as well.