[S390] set preferred console based on conmode

setup_arch() unconditionally sets the preferred console to ttyS.
This breaks the use of 3270 devices as the console. Provide a new
function to set the default preferred console for s390. The preferred
console depends on the conmode parameter that is used to switch
between 3270 and 3215 terminal/console mode.

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