[devel] Mention zlib-libpng license in each source file

scripts/makefile.elf

diff --git a/scripts/makefile.elf b/scripts/makefile.elf
index 4bee380..3e2bd9c 100644
--- a/scripts/makefile.elf
+++ b/scripts/makefile.elf
@@ -2,6 +2,8 @@
 # Copyright (C) 1998, 1999, 2002, 2006, 2008 Greg Roelofs
 # and Glenn Randers-Pehrson
 # Copyright (C) 1996, 1997 Andreas Dilger
+#
+# This code is released under the zlib-libpng license.
 # For conditions of distribution and use, see copyright notice in png.h
 
 # Modified for Debian by Junichi Uekawa and Josselin Mouette