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>
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index d668982..01a739d 100644
--- a/scripts/Makefile.autoconf
+++ b/scripts/Makefile.autoconf
@@ -6,6 +6,9 @@
 # When our migration to Kconfig is done
 # (= When we move all CONFIGs from header files to Kconfig)
 # this makefile can be deleted.
+#
+# SPDX-License-Identifier:	GPL-2.0
+#
 
 __all: include/autoconf.mk include/autoconf.mk.dep