treewide: Add SPDX license identifier - Makefile/Kconfig

Add SPDX license identifiers to all Make/Kconfig files which:

 - Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

  GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/lib/Kconfig.kgdb b/lib/Kconfig.kgdb
index ab4ff0e..bbe397d 100644
--- a/lib/Kconfig.kgdb
+++ b/lib/Kconfig.kgdb
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 
 config HAVE_ARCH_KGDB
 	bool