s390/kexec_file: Unify loader code

s390_image_load and s390_elf_load have the same code to load the different
components. Combine this functionality in one shared function.

While at it move kexec_file_update_kernel into the new function as well.

Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>