pc-bios/s390-ccw: IPL from DASD with format variations

pc-bios/s390-ccw: IPL from DASD with format variations

There are two known cases of DASD format where signatures are
incomplete or absent:

1. result of <dasdfmt -d ldl -L ...> (ECKD_LDL_UNLABELED)
2. CDL with zero keys in IPL1 and IPL2 records

Now the code attempts to
1. find zIPL and use SCSI layout
2. find IPL1 and use CDL layout
3. find CMS1 and use LDL layout
3. find LNX1 and use LDL layout
4. find zIPL and use unlabeled LDL layout
5. find zIPL and use CDL layout
6. die
in this sequence.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>