bug 833945 - provide packaged and hosted app stubs to dogfooders; r=fabrice

@fabricedesre: now that this change has approval-gaia-v1+ in [bug 833945](https://bugzilla.mozilla.org/show_bug.cgi?id=833945), can you merge it?

Note: I had to include the fix for [bug 828843](https://bugzilla.mozilla.org/show_bug.cgi?id=828843) in the pull request, because it turns out that my patch depends on it (it adds the *dogfood* MAKECMDGOAL that isolates a set of apps to dogfood builds).

cc @vingtetun and @ochameau, who worked on bug 828843, so they're aware of this merge. That bug is marked blocking-b2g-tef+, so this merge should have happened anyway.