buildroot

.gitlab-ci.yml: run our runtime tests

This commit improves our .gitlab-ci.yml logic to execute our runtime
tests located in support/testing/. To do so, this commit:

 - Adds more Debian packages to be installed, namely the nose2 and
   pexpect packages needed by the runtime testing infrastructure, as
   well as the necessary Qemu emulators

 - The description of how to run the runtime tests. Each test is
   executed as a separate Gitlab CI job, so that the status of each
   test is easily visible in the Gitlab CI web interface.

 - The Makefile is improved to auto-generate .gitlab-ci.yml from
   .gitlab-ci.yml.in, like we're doing for defconfigs. Since the
   dependencies of .gitlab-ci.yml are no longer correct, we abandon
   them and instead make it a PHONY target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Arnout: simplify .gitlab-ci.in a little, removing redundant stuff;
         make .gitlab-ci.yml a PHONY target]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>