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/boot/image-fdt.c b/boot/image-fdt.c
index 78417bd..ca51796 100644
--- a/boot/image-fdt.c
+++ b/boot/image-fdt.c
@@ -37,9 +37,9 @@
 }
 
 #if CONFIG_IS_ENABLED(LEGACY_IMAGE_FORMAT)
-static const image_header_t *image_get_fdt(ulong fdt_addr)
+static const struct legacy_img_hdr *image_get_fdt(ulong fdt_addr)
 {
-	const image_header_t *fdt_hdr = map_sysmem(fdt_addr, 0);
+	const struct legacy_img_hdr *fdt_hdr = map_sysmem(fdt_addr, 0);
 
 	image_print_contents(fdt_hdr);
 
@@ -358,7 +358,7 @@
 	switch (genimg_get_format(buf)) {
 #if CONFIG_IS_ENABLED(LEGACY_IMAGE_FORMAT)
 	case IMAGE_FORMAT_LEGACY: {
-			const image_header_t *fdt_hdr;
+			const struct legacy_img_hdr *fdt_hdr;
 			ulong load, load_end;
 			ulong image_start, image_data, image_end;