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>

tools/testing/selftests/membarrier/Makefile

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 CFLAGS += -g -I../../../../usr/include/
 
 TEST_GEN_PROGS := membarrier_test