scripts/Makefile* Add SPDX-License-Identifier tag

A general best practice for SPDX is that Makefiles should have an
identifier, add these as everything else is currently covered.

Signed-off-by: Tom Rini <trini@konsulko.com>

scripts/Makefile.extrawarn

@@ -9,6 +9,9 @@
 # $(call cc-option, -W...) handles gcc -W.. options which
 # are not supported by all versions of the compiler
 # ==========================================================================
+#
+# SPDX-License-Identifier:	GPL-2.0
+#
 
 ifeq ("$(origin W)", "command line")
   export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)