pc-bios/s390-ccw: add boot entry selection for ECKD DASD

1. change a bit definition of ScsiMbr to allow an array of pointers
2. add loadparm fetch to boot script processing
3. apply loadparm index to boot entry selection, if any

Initial patch from Eugene (jno) Dvurechenski.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>