Test all targets on all releases

Added a test to at least try to install all targets on all supported releases.

Then proceeded to fix the problems the test triggered. The target files exit with code 99 when a target/release/architecture is not supported, so that the test can easily detect it.

I'm not fully happy about the test itself (39-alltargets):
 1. It takes a very long time to run.
 2. It is not very smart (e.g. not all targets depend on x11).
 3. It does not test functionality (for DE/WM, once we have x11 testing, we'd like to actually start the DE/VM and, at least, verify it does not crash).
I'm thinking about reverting that commit for now, and leave it in a branch/history so that we can pick it later (or when needed).

Any opinion?

Fixes #298, #997, and a few others we did not notice (unity, chromium).
Sorry, I'm apparently new to this whole crouton thing.