[POWERPC] bootwrapper: Use physical address in PHDR for uImage

Now that we properly set the physical address in the program header of the
vmlinux ELF we can extract it to properly set the load and entry point for
u-boot uImages.  Before we always hard coded the load & entry point to 0.
However there are situations that the kernel may be built with a non-zero
physical address.

We use objdump to extract the PHDR.  We assume that there is only one
PHDR in the vmlinux of type LOAD.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>