s390: Rework kernel loading: supports elf and newer kernels

This reworks the image loading on s390.

Newer kernels will not always have a 0dd0 (basr 13,0) at address 0x10000.
We must not rely on specific code at certain  addresses. This check was
introduced to warn users that tried to load vmlinux, since ELF loading
was not supported. Lets wire that up. If elf loading fails, we assume
that this is a standard kernel image and load that via load_image_targphys.
This patch also changes all other users of load_image to
load_image_targphys to be consistent. (the elf loader registers the kernel
as rom).

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>