assemble.py: Add explicit license declaration

Although this file is likely implicitly licensed under the Apache 2.0
license because of the LICENSE file for this project, make this explicit
in this file.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/assemble.py b/scripts/assemble.py
index 30adcba..7a38985 100755
--- a/scripts/assemble.py
+++ b/scripts/assemble.py
@@ -1,4 +1,18 @@
 #! /usr/bin/env python3
+#
+# Copyright 2017 Linaro Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
 
 """
 Assemble multiple images into a single image that can be flashed on the device.