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>

board/ti/ks2_evm/board.c

@@ -64,9 +64,9 @@ int dram_init(void)
 	return 0;
 }
 
-struct image_header *spl_get_load_buffer(ssize_t offset, size_t size)
+struct legacy_img_hdr *spl_get_load_buffer(ssize_t offset, size_t size)
 {
-	return (struct image_header *)(CONFIG_SYS_TEXT_BASE);
+	return (struct legacy_img_hdr *)(CONFIG_SYS_TEXT_BASE);
 }
 
 int board_init(void)