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>

lib/xz/Kconfig

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 config XZ_DEC
 	tristate "XZ decompression support"
 	select CRC32

lib/xz/Makefile

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 obj-$(CONFIG_XZ_DEC) += xz_dec.o
 xz_dec-y := xz_dec_syms.o xz_dec_stream.o xz_dec_lzma2.o
 xz_dec-$(CONFIG_XZ_DEC_BCJ) += xz_dec_bcj.o