powerpc/CoreNet: add tool to support pbl image build.

Provides a tool to build boot Image for PBL(Pre boot loader) which is
used on Freescale CoreNet SoCs, PBL can be used to load some instructions
and/or data for pre-initialization. The default output image is u-boot.pbl,
for more details please refer to doc/README.pblimage.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>