s390/boot: Fix startup debugging log

Fix 'kernel image' end address for kaslr case.

Fixes: ec6f9f7e ("s390/boot: Add startup debugging support")
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>