tools: default_image: Accept images with padding

If image file is stored on flash partition then it contains padding, which
is not part of the image itself. Image data size is stored in the image
header. So use image size from the header instead of expecting that total
image file size is size of the header plus size of the image data. This
allows dumpimage to parse image files with padding (e.g. dumped from flash
partition).

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>