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. In case $JAVA_HOME is empty string and /bin/java exists the condition was not behaving as intended.

I did not test the windows bat file, but the change is pretty small so should not cause any problems.