boot: Rename fit_image_get_data_and_size()

This function is really just getting the data. The size comes along for
the ride. In fact this function is only reliable way to obtain the data
for an image in a FIT, since the FIT may use external data.

Rename it to fit_image_get_data()

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/ximg.c b/cmd/ximg.c
index 1c96f5a..29d7c32 100644
--- a/cmd/ximg.c
+++ b/cmd/ximg.c
@@ -161,8 +161,7 @@
 		}
 
 		/* get subimage/external data address and length */
-		if (fit_image_get_data_and_size(fit_hdr, noffset,
-					       &fit_data, &fit_len)) {
+		if (fit_image_get_data(fit_hdr, noffset, &fit_data, &fit_len)) {
 			puts("Could not find script subimage data\n");
 			return 1;
 		}