Update and clean jcr2vfs migration app scripts

* remove the droolsjbpm-uberjar from CP in script as it will be loaded
   by the "../libs/*" pattern and placing it before the other libs won't
   result in loading it first
 * fix condition that checks if $JAVA_HOME/bin/java exists - first check
   if $JAVA_HOME is not empty string and the check if $JAVA_HOME/bin/java exists

I did not test the windows bat file, but the change is pretty small so should not cause any problems.