utils/test-pkg: remove configurations that are skipped

utils/test-pkg: remove configurations that are skipped

When the config fragment provided by the user is not usable with a
specific toolchain configuration, the resulting .config file was kept
around.

In a follow up commit, we'll need to know, from outside test-pkg, if a
specific configuration was indeed usable or not.

So, unless if the user actually requested to keep the build directories,
remove the .config file when it contains a configration that would be
skipped.

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: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>