It is sometimes useful to have an area of the image which is all zeroes,
or all 0xff. This can often be achieved by padding the size of an an
existing entry and setting the pad byte for an entry or image.

But it is useful to have an explicit means of adding blocks of repeating
data to the image. Add a 'fill' entry type to handle this.
Signed-off-by: NSimon Glass <sjg@chromium.org>