[S390] s390/setup: set default preferred console device "ttyS"

This patch sets the default console device for s390.

The console= kernel parameter can be still used to switch the preferred
console to some other device. In that case, console messages are also
printed on the default console device (ttyS0).

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>