utils/test-pkg: add mode to only prepare .config files

utils/test-pkg: add mode to only prepare .config files

Currently, running test-pkg is only done locally on the developers
machine.

In a follow up commit, we'll add the possibility to run test-pkg in a
gitlab-ci pipeline and, to speed things up, with one job per buildable
configuration.

As such, we will need to make sure that test-pkg only prepares the
configurations, and that it does not build them.

Add such a mode, with a new option, --prepare-only

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>