Add "make test" and custom packages
The purpose of these changes are to enable some basic automation of QA checking on image builds.
- Add "make test" target, currently only tests on virtualbox. It builds the virtualbox image, creates and configures a VM, boots it and waits for first-run to complete. Then it runs plinth diagnostics, and logs the results to a file.
- Allow custom plinth and freedombox-setup packages to be specified from the make command. This is to test changes that haven't been uploaded to Debian yet.