tools: mkimage: Add StarFive SPL image support

The StarFive JH7110 base boards require a header to be prefixed to the SPL
binary image. This has previously done with a vendor tool 'spl_tool'
published under a GPL-2-or-later license. Integrate this capability into
mkimage.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Chanho Park <chanho61.park@samsung.com>
Tested-by: Milan P. Stanić <mps@arvanta.net>
diff --git a/include/image.h b/include/image.h
index 01a6787..5f85bf8 100644
--- a/include/image.h
+++ b/include/image.h
@@ -231,6 +231,7 @@
 	IH_TYPE_SUNXI_TOC0,		/* Allwinner TOC0 Boot Image */
 	IH_TYPE_FDT_LEGACY,		/* Binary Flat Device Tree Blob	in a Legacy Image */
 	IH_TYPE_RENESAS_SPKG,		/* Renesas SPKG image */
+	IH_TYPE_STARFIVE_SPL,		/* StarFive SPL image */
 
 	IH_TYPE_COUNT,			/* Number of image types */
 };