travis.yml: Test the release tarball

travis.yml: Test the release tarball

Add a job to generate the release tarball and build/install few
QEMU targets from it.

Ideally we should build the 'efi' target from the 'roms' directory,
but it is too time consuming.

This job is only triggered when a tag starting with 'v' is pushed,
which is the case with release candidate tags.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191007160450.3619-1-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>