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>

cmd/source.c

@@ -46,7 +46,7 @@ int image_source_script(ulong addr, const char *fit_uname)
 {
 	ulong		len;
 #if defined(CONFIG_LEGACY_IMAGE_FORMAT)
-	const image_header_t *hdr;
+	const struct legacy_img_hdr *hdr;
 #endif
 	u32		*data;
 	int		verify;