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/arch/x86/lib/bootm.c b/arch/x86/lib/bootm.c
index 4fdf1b2..eafcddf 100644
--- a/arch/x86/lib/bootm.c
+++ b/arch/x86/lib/bootm.c
@@ -72,7 +72,7 @@
 static int boot_prep_linux(struct bootm_headers *images)
 {
 	char *cmd_line_dest = NULL;
-	image_header_t *hdr;
+	struct legacy_img_hdr *hdr;
 	int is_zimage = 0;
 	void *data = NULL;
 	size_t len;