Fix build errors when building in different artifactdir to recipedir

If building images in a different artifactdir from the recipedir, for instance: $ debos --artifactdir=out image-uboot.yaml

the postprocessing steps which compress the images fail.

Fix the postprocessing step to save the resultant artifacts inside the artifactdir rather than assuming the current directory is where the artifacts should be saved.

Signed-off-by: Christopher Obbard chris.obbard@collabora.com