examples: Remove padding of update images

The update image should not be padded to fill the whole slot, because
the image to download is smaller without padding, and padding would
overwrite the existing the trailer area which keeps track of boot and
update statuses.

The initial image remains padded, because the trailer area needs to be
in a clean state during initial boot.

All credit to Vincent Coubard <vincent.coubard@arm.com> who proposed
this change.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>