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.uncmd_spl b/scripts/Makefile.uncmd_spl
index 4003546..15d0836 100644
--- a/scripts/Makefile.uncmd_spl
+++ b/scripts/Makefile.uncmd_spl
@@ -1,5 +1,7 @@
 # Makefile version of include/config_uncmd_spl.h
 #
+# SPDX-License-Identifier:	GPL-2.0+
+#
 # TODO: Invent a better way
 
 ifdef CONFIG_SPL_BUILD