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/common/spl/spl_xip.c b/common/spl/spl_xip.c
index e9a40b0..1258d85 100644
--- a/common/spl/spl_xip.c
+++ b/common/spl/spl_xip.c
@@ -25,6 +25,6 @@
 	}
 #endif
 	return(spl_parse_image_header(spl_image, bootdev,
-	       (const struct image_header *)CONFIG_SYS_UBOOT_BASE));
+	       (const struct legacy_img_hdr *)CONFIG_SYS_UBOOT_BASE));
 }
 SPL_LOAD_IMAGE_METHOD("XIP", 0, BOOT_DEVICE_XIP, spl_xip);