spapr: support only ELF kernel images

Currently everybody uses ELF kernel images with "-kernel" option on
pseries machine but QEMU still tries to boot from an image even it
fails to recognize it is ELF. This produces undefined behaviour if
the user tries a kernel image compiled for another architecture.

This removes support of raw kernel images.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
moxie: fix load_elf() usage

At the moment in the case of error, load_elf() returns -1 so load_kernel()
will not signal error at all.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
elf-loader: add more return codes

The existing load_elf() just returns -1 if it fails to load ELF. However
it could be smarter than this and tell more about the failure such as
wrong endianness or incompatible platform.

This adds additional return codes for wrong architecture, wrong
endianness and if the image is not ELF at all.

This adds a load_elf_strerror() helper to convert return codes into
string messages.

This fixes handling of what load_elf() returns for s390x, other
callers just check the return value for <0 and this remains unchanged.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
spapr: print more detailed error message on failed load_elf()

This makes use of new error codes which load_elf() can return and
prints more informative error message.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>