rockchip: Drop first 32kb of zeros from the rkSD image type

Instead of creating a rockchip SPL SD card image with 32KB of zeros
which can be written to the start of an SD card, create the images with
only the useful data that should be written to an offset of 32KB on the
SD card.

The first 32 kilobytes aren't needed for bootup and only serve as
convenient way of accidentally obliterating your partition table.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>