Phil!!!! Thank you for this! It works. I had asked at multiple places about this topic and no one had a clue. Google doesn't find this thread for some reason.
I updated your instructions a bit to match what I just had to do from Retropie 4.2........
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>
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
5f- You should then see your custom splashscreen folder, choose the file you added as your splashscreen.
5g- 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
Here is a quick vid of it booting up, no boot text, straight into the rom.https://www.youtube.com/watch?v=5cfR2VTzPI0