s390/configs: provide compat topic configuration target

CONFIG_COMPAT is disabled by default, however compat code still needs to
be compile tested. Add a compat topic configuration target which allows to
enable it easily.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>