since last update openhab seemed not to start up, when used with systemd – it starts up and shuts down immediatly-. However it worked when started from a terminal. So a stupid workaround:

yaourt -S tmux

cat /etc/systemd/system/openhab.service

[Unit] Description=OpenHAB runtime 
After=syslog.target network.target 
[Service] 
Type=forking 
#EnvironmentFile=-/etc/conf.d/openhab 
WorkingDirectory=/opt/openhab2 
ExecStart=/usr/bin/tmux new-session -s openHAB -d "/usr/bin/sh ./start.sh -Djava.awt.headless=true -Dgnu.io.rxtx.SerialPorts=/dev/ttyUSB1" 
ExecStop=/usr/bin/tmux send-keys -t openHAB "logout" Enter # 
[Install] 
WantedBy=multi-user.target

modified openhab service file - for startup issues