checkstyle: Use $config_loc for Java 11

Also move configuration files into "${rootProject}/config/checkstyle"
to follow convention over configuration.

See https://github.com/gradle/gradle/issues/8286#issuecomment-458145619

config/checkstyle/header.txt

@@ -0,0 +1,16 @@
+/**
+ *
+ * Copyright 20XX the original author or authors
+ *
+ * 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.
+ */