binman: Make section padding consistent with other entries

At present padding of sections is inconsistent with other entry types, in
that different pad bytes are used.

When a normal entry is padded by its parent, the parent's pad byte is
used. But for sections, the section's pad byte is used.

Adjust logic to always do this the same way.

Note there is still a special case in entry_Section.GetPaddedData() where
an image is padded with the pad byte of the top-level section. This is
necessary since otherwise there would be no way to set the pad byte of
the image, without adding a top-level section to every image.

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