image: Add support for Plan 9

Signed-off-by: Steven Stallion <sstallion@gmail.com>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/include/image.h b/include/image.h
index 8e285f9..4ad0e6b 100644
--- a/include/image.h
+++ b/include/image.h
@@ -84,6 +84,7 @@
 #define IH_OS_UNITY		20	/* Unity OS	*/
 #define IH_OS_INTEGRITY		21	/* INTEGRITY	*/
 #define IH_OS_OSE		22	/* OSE		*/
+#define IH_OS_PLAN9		23	/* Plan 9	*/
 
 /*
  * CPU Architecture Codes (supported by Linux)