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/arch/arm/net/Makefile b/arch/arm/net/Makefile
index c2c1084..3354b3e 100644
--- a/arch/arm/net/Makefile
+++ b/arch/arm/net/Makefile
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 # ARM-specific networking code
 
 obj-$(CONFIG_BPF_JIT) += bpf_jit_32.o