roslaunch: Use as documented environment

in relation to #307 

roscore-default: Removing not supported variable in variable
roscore-default: Adding PATH, LD_LIBRARY_PATH and PYTHONPATH
roscore.service: Using /opt/ros/hydro/bin instead of /usr/bin to launch
roscore

You can now install roslaunch-systemd :

opkg install roslaunch-systemd

and start roscore :

systemctl start roscore
Remove this line, and restore old one instead (see above)