uboot-imx

rockchip: mkimage: use imagename to select spl hdr & spl size

Our chips may have different spl size and spl header, so
use imagename(passed by "mkimage -n") to select them now.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>