classes: locate packages in /opt/ros/${ROSDISTRO}

Following standard convention, place ROS/Catkin packages under the
prefix /opt/ros/${ROSDISTRO} (exported as ${ros_prefix}) rather than in
the usual ${prefix}.
README.md: update to reflect new ROS package prefix

Update the setup instructions to reflect the use of ${ros_prefix}.
ros_realtime_tests: patch to fix library build path issue

Adds a patch to ros_realtime_tests to properly export and import the
rt_tests_support library.