binman: Add info to allow safely repacking an image later

At present it is not possible to discover the contraints to repacking an
image (e.g. maximum section size) since this information is not preserved
from the original image description.

Add new 'orig-offset' and 'orig-size' properties to hold this. Add them to
the main device tree in the image.

Signed-off-by: Simon Glass <sjg@chromium.org>