doc: rockchip: document packing second level loader with mkimage

Add documentation about packing optional second level boot-loader with
mkimage tool.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/doc/README.rockchip b/doc/README.rockchip
index ffab8ff..9b699b9 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -340,6 +340,12 @@
    cat firefly-rk3288/u-boot-dtb.bin >> out && \
    sudo dd if=out of=/dev/sdc seek=64
 
+Or:
+   ./firefly-rk3288/tools/mkimage -n rk3288 -T rksd -d \
+	firefly-rk3288/spl/u-boot-spl-dtb.bin:firefly-rk3288/u-boot-dtb.bin \
+	out && \
+   sudo dd if=out of=/dev/sdc seek=64
+
 If you have an HDMI cable attached you should see a video console.
 
 For evb_rk3036 board:
@@ -347,6 +353,11 @@
 	cat evb-rk3036/u-boot-dtb.bin >> out && \
 	sudo dd if=out of=/dev/sdc seek=64
 
+Or:
+	./evb-rk3036/tools/mkimage -n rk3036 -T rksd -d \
+		evb-rk3036/spl/u-boot-spl.bin:evb-rk3036/u-boot-dtb.bin out && \
+	sudo dd if=out of=/dev/sdc seek=64
+
 Note: rk3036 SDMMC and debug uart use the same iomux, so if you boot from SD, the
       debug uart must be disabled