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_ext.c b/common/spl/spl_ext.c
index ebd914c..f117c63 100644
--- a/common/spl/spl_ext.c
+++ b/common/spl/spl_ext.c
@@ -15,7 +15,7 @@
 		       const char *filename)
 {
 	s32 err;
-	struct image_header *header;
+	struct legacy_img_hdr *header;
 	loff_t filelen, actlen;
 	struct disk_partition part_info = {};
 
@@ -41,7 +41,7 @@
 		puts("spl: ext4fs_open failed\n");
 		goto end;
 	}
-	err = ext4fs_read((char *)header, 0, sizeof(struct image_header), &actlen);
+	err = ext4fs_read((char *)header, 0, sizeof(struct legacy_img_hdr), &actlen);
 	if (err < 0) {
 		puts("spl: ext4fs_read failed\n");
 		goto end;