openocd: src: replace the GPL-2.0-or-later license tag

Replace the FSF boilerplate with the SPDX tag.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: I30cd66ac7d737f1973c68fdbb841ffcf00e917c4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7072
Tested-by: jenkins

src/server/rtt_server.h

@@ -1,18 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
 /*
  * Copyright (C) 2016-2017 by Marc Schink <dev@zapb.de>
- *
- * This program 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.
- *
- * This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #ifndef OPENOCD_SERVER_RTT_SERVER_H