PPC: e500: Fix duplicate kernel load and device tree overlap

PPC: e500: Fix duplicate kernel load and device tree overlap

This patch fixes an incorrect behavior when the -kernel argument has been
specified without -bios. In this case the kernel was loaded twice. At address
32M as a raw image and afterwards by load_elf/load_uimage at the
corresponding load address. In this case the region for the device tree and
the raw kernel image may overlap.

The patch fixes the behavior by loading the kernel image once with
load_elf/load_uimage and skips loading the raw image.

When here do not use bios_name/size for the kernel and use a more generic
name called payload_name/size.

New in v3: dtb must be stored between kernel and initrd because Linux can
           handle the dtb only within the first 64MB. Add a comment to
           clarify the behavior.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>