SPL: Add option to skip copying of the mkimage header

On some system (e.g. powerpc), the load-address and entry-point is
located at address 0. So the current approach to load the image
(payload) including the header to the address "load-address - 64"
can't work here.

This patch adds an flag to skip this copying including header to
the SPL framework. By setting SPL_COPY_PAYLOAD_ONLY, only the
playload will be copied. This will be used by the SPL NOR flash
driver on powerpc.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>