rockchip: Add support for the SD image

The Rockchip boot ROM requires a particular file format. It consists of
64KB of zeroes, a 512-byte header encoded with RC4, and then some executable
code.

Add support to mkimage so that an SPL image (u-boot-spl-dtb.bin) can be
converted to this format.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/image.h b/include/image.h
index 2daa74a..ea16205 100644
--- a/include/image.h
+++ b/include/image.h
@@ -246,8 +246,9 @@
 #define IH_TYPE_LPC32XXIMAGE	21	/* x86 setup.bin Image		*/
 #define IH_TYPE_LOADABLE	22	/* A list of typeless images	*/
 #define IH_TYPE_RKIMAGE		23	/* Rockchip Boot Image		*/
+#define IH_TYPE_RKSD		24	/* Rockchip SD card		*/
 
-#define IH_TYPE_COUNT		24	/* Number of image types */
+#define IH_TYPE_COUNT		25	/* Number of image types */
 
 /*
  * Compression Types