Remove display system specific environment
We need the prelaunch daemon to run in both X and wayland environments,
so directly exporting DISPLAY or ELM_ENGINE in the service file will
not work. Instead the service file will use the EnvironmentFile mechanism
to load a snippet (if the file exists).
Each vertical can create a /etc/sysconfig/wrt file containing key/value
pairs that needed for the specific environment. This could be done in the
post section of a ks file or any other mechanism.Z
Change-Id: I8e14233595c6e992b718ae9dfa14077a026c657c