cm: bootanimation: Clear intermediaries

Make sure that any changes to the boot animation can be picked up
and rebuilt by running 'mka bootanimation.zip'.

Change-Id: Ice10e919df4c1b651c5c5dbb9700cab38eeac748
(cherry picked from commit e796a286bd72df3e4715164171a3e7ef6d2eadb2)

bootanimation/CleanSpec.mk

@@ -0,0 +1,17 @@
+#
+# Copyright (C) 2017 The LineageOS Project
+#
+# 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.
+#
+
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/BOOTANIMATION)