s390x/ipl: Fix crash of ELF images with arbitrary entry points

When loading S390 kernels, the current code expects an ELF file with the
start address 0x10000. Other ELF files cause a segmentation fault. To avoid
these crashes, we should get the start address from the ELF file instead
of always using a hard-coded address.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
