treewide: Drop image_header_t typedef

This is not needed and we should avoid typedefs. Use the struct instead
and rename it to indicate that it really is a legacy struct.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/mkimage.c b/tools/mkimage.c
index 597cb3a..30c6df7 100644
--- a/tools/mkimage.c
+++ b/tools/mkimage.c
@@ -845,7 +845,7 @@
 	if (params.xflag) {
 		unsigned char *p = NULL;
 		/*
-		 * XIP: do not append the image_header_t at the
+		 * XIP: do not append the struct legacy_img_hdr at the
 		 * beginning of the file, but consume the space
 		 * reserved for it.
 		 */