As you wish herbfargus, here is the solution.

1. go to console (f4 to quit emulationstation), or use a program of your chois on your pc (in my case i used PuTTY)
2. log in with your user and password
3. enter cd /opt/retropie/configs/all/
4. open with sudo nano retroarch.cfg the retroarch.cfg file
5. go to the end of the file and insert input_enable_hotkey_btn = "16" for the ps-button or another number (contoller) or letter (keyboard) for the button you of your wish.
6. Then press “ctrl-x” to exit the file, “y” (without quotes) to safe and press “Enter” to safe it as retroarch.cfg

And thats it.

BTW: herbfargus, i said CLOSE or Delet it ;)