pc-bios/s390-ccw: El Torito s390x boot entry check

Boot entry is considered compatible if boot image is Linux kernel
with matching S390 Linux magic string.

Empty boot images with sector_count == 0 are considered broken.

Signed-off-by: Maxim Samoylov <max7255@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
