s390: set bzImage as default image for packaging

Even though s390 compressed kernel support has been introduced long ago,
by default uncompressed kernel image is used by packaging targets like:
"make rpm-pkg", "make deb-pkg" as well as "make image_name".

At the same time "make install" would use compressed bzImage by default.
To avoid ambiguity, set bzImage as default image for packaging.

Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>