cmd: ximg.c: Add support for getting external data address and length

This function supports getting both data address and length for
existing FIT subimage and FIT external data.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
diff --git a/cmd/ximg.c b/cmd/ximg.c
index 8572a67..32bfae8 100644
--- a/cmd/ximg.c
+++ b/cmd/ximg.c
@@ -159,9 +159,9 @@
 			}
 		}
 
-		/* get subimage data address and length */
-		if (fit_image_get_data(fit_hdr, noffset,
-					&fit_data, &fit_len)) {
+		/* get subimage/external data address and length */
+		if (fit_image_get_data_and_size(fit_hdr, noffset,
+					       &fit_data, &fit_len)) {
 			puts("Could not find script subimage data\n");
 			return 1;
 		}