utils/test-pkg: clean output dir for successful builds

utils/test-pkg: clean output dir for successful builds

test-pkg will use gigabytes of space when testing all toolchains.
Nevertheless, you are normally only interested in the actual build / host
tree when there is a build failure.

Do a 'make clean' for successful builds to save disk space, unless the new
option '-k/--keep' is set.
Note that the logfile and configuration is always retained for inspection.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>