s390/decompressor: allow to pack uncompressed vmlinux.bin into piggy.o

If none of compression methods defined, leave suffix-y empty, so that
plain uncompressed vmlinux.bin could be reused instead.

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