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>

arch/mips/bcm63xx/boards/Makefile

@@ -1 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
 obj-$(CONFIG_BOARD_BCM963XX)		+= board_bcm963xx.o