scripts: Add Layerscape image tool ...

This adds the Layerscape image tool based on the corresponding U-Boot
tool. PBL in the name is for "Pre-Boot Loader", just like the barebox
PBL support, but here the name is for the Layerscape image format which
has the same name.

Layerscape SoCs load the initial image portion into SRAM, so only a
fraction of the barebox image can initially be loaded. This tool expects
the full barebox image and the size of the PBL part of this image
specified to the -m option. The full barebox image is then placed in the
output image at offset 128KiB.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>