modify to work under Linux

scripts/sha256sum

@@ -2,6 +2,10 @@
 # This file is part of the OpenADK project. OpenADK is copyrighted
 # material, please see the LICENCE file in the top-level directory.
 
-tmp=$(mktemp -t yyy)
-cat - > $tmp
-shasum -a 256 "$@" $tmp
+if [ -x /usr/bin/sha256sum ]; then
+	/usr/bin/sha256sum "$@"
+else
+	tmp=$(mktemp -t yyy)
+	cat - > $tmp
+	shasum -a 256 "$@" $tmp
+fi