Add GPL-2.0+ SPDX-License-Identifier to source files

Signed-off-by: Wolfgang Denk <wd@denx.de>
[trini: Fixup common/cmd_io.c]
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/fs/zfs/zfs.c b/fs/zfs/zfs.c
index 1b73244..099d517 100644
--- a/fs/zfs/zfs.c
+++ b/fs/zfs/zfs.c
@@ -8,19 +8,7 @@
  *	Free Software Foundation, Inc.
  *	Copyright 2004	Sun Microsystems, Inc.
  *
- *	GRUB is free software; you can redistribute it and/or modify
- *	it under the terms of the GNU General Public License as published by
- *	the Free Software Foundation; either version 2 of the License, or
- *	(at your option) any later version.
- *
- *	GRUB is distributed in the hope that it will be useful,
- *	but WITHOUT ANY WARRANTY; without even the implied warranty of
- *	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *	GNU General Public License for more details.
- *
- *	You should have received a copy of the GNU General Public License
- *	along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
- *
+ * SPDX-License-Identifier:	GPL-2.0+
  */
 
 #include <common.h>