Makefile: rockchip: Support SPL-alone mkimage

Add SPL-alone mkimage tooling support via Makefile for
few platforms or boards used in rockchip family.

With this users would get rid of explicitly creating
mkimage tool for rockchip rksd or rkspi boot modes.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>