just learned how to get Retropie to load to just one game and confirmed it works.
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Gleaned from Philby95 on BYOC:
Updated for Retropie 4.2 release from March 2017
Here is another way to boot into a single rom on a pi.
1- Use win32diskimager to install retropie on your sd card.
1a- Setup retropie as per normal configuring controllers and adding roms etc.
2- Start Retropie to load Emulation Station, hit F4 to exit to the retropie command prompt.
3- Type> sudo nano /boot/cmdline.txt
3a- Note- When this opens you should see one line of commands, it must stay as one line.
3b- Look for console=tty1 replace it with> console=tty3
3c- Confirm you have this toward the end of the line> quiet loglevel=3
3d- Add to the end of the line> logo.nologo
3e- Hit Control X to bring up the Save command
3f- Hit Y then hit Enter to overwrite the file
4- Type> sudo nano /boot/config.txt
4a- Scroll to bottom of file add the following lines>
disable_splash=1
avoid_warnings=1
4b- Note- this will stop the raspberry temperture icon showing during boot.
5- Enable splashscreens
5a- Note- The Splash Screen Menu can be accessed from the RetroPie Menu in EmulationStation or through the setup script under option 3.
5b- Navigate to Retropie Menu in EmulationStation
5c- Choose Splashscreens
5d- Use WinSCP from your computer to put a .png image of the desired game in the /home/pi/RetroPie/splashscreens
5e- Select Own Splashscreen in the menu
5d- You should then see your custom splashscreen folder, choose the file you added as your splashscreen.
5e- Reboot and hopefully you should see no boot messages
6- Setup Booting into a Single Rom
6a- Optional methods- Login to the Pi using Putty from your computer or F4 from the Retropie menu
6b- Type> sudo nano /opt/retropie/configs/all/autostart.sh
6c- Add as the top line>
/opt/retropie/supplementary/runcommand/runcommand.sh 0 _SYS_ mame-mame4all ~/RetroPie/roms/arcade/romname.zip
6d- Optional- If you want to boot into a game running MAME2016 instead. Make sure that lr-mame2016 is the only "lr-" version of mame installed on the pi.
Use this as the top line>
/opt/retropie/supplementary/runcommand/runcommand.sh 0 _SYS_ mame-libretro ~/RetroPie/roms/arcade/romname.zip
Note- If you want to lock out any pi menus and force the player to stay in just one game, simply don't map the "UI Cancel" (found in MAME Config via TAB) to any available buttons on the control panel.
Here is a quick vid of it booting up, no boot text, straight into the rom.
https://www.youtube.com/watch?v=5cfR2VTzPI0