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.build b/scripts/Makefile.build
index de818ae..06e5c20 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -1,6 +1,9 @@
 # ==========================================================================
 # Building
 # ==========================================================================
+#
+# SPDX-License-Identifier:	GPL-2.0
+#
 
 # Modified for U-Boot
 prefix := tpl