LUCENE-5355: Add support for -Dbootjdk to point to a separate JAVA_HOME that is used to generate javadocs; validate the -Dbootclasspath to point to a valid rt.jar

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1547346 13f79535-47bb-0310-9956-ffa450edef68