Acceptance tests: add make targets to download images

The newly introduced "boot linux" tests make use of Linux images that
are larger than usual, and fall into what Avocado calls "vmimages",
and can be referred to by name, version and architecture.

The images can be downloaded automatically during the test. But, to
make for more reliable test results, this introduces a target that
will download the vmimages for the architectures that have been
configured and are available for the currently used distro (Fedora
31).

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
[Cleber: implemented suggestions by Alex, download message, check-venv target]
Message-Id: <20200317141654.29355-4-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>