kconfig: convert to SPDX License Identifier

All files in lxdialog/ are licensed under GPL-2.0+, and the rest are
under GPL-2.0. I added GPL-2.0 tags to test scripts in tests/.

Documentation/process/license-rules.rst does not suggest anything
about the flex/bison files. Because flex does not accept the C++
comment style at the very top of a file, I used the C style for
zconf.l, and so for zconf.y for consistency.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/nconf.h

@@ -1,9 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
- * Copyright (C) 2008 Nir Tzachar <nir.tzachar@gmail.com?
- * Released under the terms of the GNU GPL v2.0.
+ * Copyright (C) 2008 Nir Tzachar <nir.tzachar@gmail.com>
  *
  * Derived from menuconfig.
- *
  */
 
 #include <ctype.h>