I actually just set this up last night. These are the steps I took:
1) in /opt/retropie/configs/all/retroarch.cfg – edit the file and add this piece of code:
input_menu_toggle_btn = “<your button choice>”
This will allow you to directly enter the menu while playing a rom.
2) When in an emulator open up the menu by pushing the button you assigned above (you will need to hold in the enable hotkey button as well)
*please note, if you do not have the left analog enabled you will need to use a keyboard for this part. when I did this I had to use both the controller and the keyboard to navigate through the menu*
3) when in the menu go to Settings >> Input Options. Scroll down and program the following buttons:
Left analog X+ (right)
Left analog X- (left)
Left analog Y+ (down)
Left analog Y- (up)
Fast Forward toggle
If you click the A button you will be able to input these controls by pressing the corresponding button on your controller. (ie press A, then when prompted press right on the d-pad to enable Left analog X+).
The analog buttons are crucial so you can navigate through the menu without having to use the keyboard. The Fast Forward Toggle will do what you are looking for.
THIS IS IMPORTANT –> press back to get to the first menu and SAVE the configs (there is an option to do so on the bottom of the first menu screen).
WHAT THIS DOES: Saves a new configuration to the /opt/retropie/configs/all/ file path.
4) Exit Emulation Station and go back to the command line. From here, go to the file path where the retroarch.cfg is saved. At this point I recommend making a copy of that file and naming it something different (retroarchBACKUP.cfg or something).
5) the file that you made in the emulationstation menu should be called libretro.cfg or something, RENAME this file to be retroarch.cfg. This will ensure that every time you boot up the emulationstation, the changes you made are the default controller (ie you can navigate through the menu and you can enable fast forward).
save and exit. Next time you enter emulation station this should work.
You can just do this all manually in the retroarch.cfg (adding in all the proper button toggles and such). If you would like I can share some code with you but I will need to boot up my pie, currently I’m just typing this from memory.