Since the newly introduced image-artifact-names class is inherited at later
processing stage, individual IMAGE_NAME settings are overridden (hopefully
by accident instead on purpose).
Allow derived distributions define their own name schema by setting defaults
iwth question mark.
Signed-off-by: Jens Rehsack <sno@netbsd.org> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
