Use JAVA_HOME from environment if present

This change causes the rules file to set JAVA_HOME to OpenJDK only if
the variable is not already set in the environment.

A change to follow this one will allow the build to depend on any
compatible JDK rather than on OpenJDK specifically.

Change-Id: I9c238d2de67300891dc8274b97b2b3ee0e5d79ca
Merge "Use JAVA_HOME from environment if present" into debian