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/tools/Makefile b/tools/Makefile
index 267f55c..9082bda 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -64,7 +64,7 @@
 					rsa-sign.o rsa-verify.o rsa-checksum.o \
 					rsa-mod-exp.o)
 
-ROCKCHIP_OBS = lib/rc4.o rkcommon.o rkimage.o
+ROCKCHIP_OBS = lib/rc4.o rkcommon.o rkimage.o rksd.o
 
 # common objs for dumpimage and mkimage
 dumpimage-mkimage-objs := aisimage.o \