splash: add support for loading splash from a FIT image

Enable support for loading a splash image from within a FIT image.
The image is assumed to be generated with mkimage -E flag to hold
the data external to the FIT.

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/include/splash.h b/include/splash.h
index 136eac7..228aff4 100644
--- a/include/splash.h
+++ b/include/splash.h
@@ -33,8 +33,9 @@
 };
 
 enum splash_flags {
-	SPLASH_STORAGE_RAW,
-	SPLASH_STORAGE_FS,
+	SPLASH_STORAGE_RAW, /* Stored in raw memory */
+	SPLASH_STORAGE_FS,  /* Stored within a file system */
+	SPLASH_STORAGE_FIT, /* Stored inside a FIT image */
 };
 
 struct splash_location {