Run raspi-config and enable i2c in advanced options

$ sudo apt-get install python-smbus

$ sudo i2cdetect -y 0 $ sudo i2cdetect -y 1

in /opt/retropie/configs/all/runcommand-onstart.sh add python /home/pi/retropie_lcd/retropie_lcd.py $1 $2 $3

in /opt/retropie/configs/all/runcommand-onstart.sh add python /home/pi/retropie_lcd/retropie_lcd.py

i2c_lib and lcd_display.py are taken from the raspi-gpio package by Paul Barber

Thanks to Paul Barber for creating raspi-gpio