cbfs: Rename checksum to attributes_offset

It seems that this field has been renamed in later version of coreboot.
Update it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/cbfs.h b/include/cbfs.h
index f2ede25..b8d1dab 100644
--- a/include/cbfs.h
+++ b/include/cbfs.h
@@ -65,7 +65,8 @@
 	u8 magic[8];
 	u32 len;
 	u32 type;
-	u32 checksum;
+	/* offset to struct cbfs_file_attribute or 0 */
+	u32 attributes_offset;
 	u32 offset;
 } __packed;
 
@@ -76,7 +77,7 @@
 	u32 data_length;
 	char *name;
 	u32 name_length;
-	u32 checksum;
+	u32 attributes_offset;
 } __packed;
 
 extern enum cbfs_result file_cbfs_result;