Here is the code for launching to MAME emulator only:
1. from command line type:
sudo nano /etc/rc.local
2. add the following text to the bottom:
sudo /opt/retropie/emulators/mame4all-pi/mame
3. Save the file and reboot.
I looked all over for this answer and most of the things I found were out of date, so specific folders or files were not where the forums directed. I got it to work for MAME using the code above; it puts the script for launching MAME emulator from command line into the rc.local file. I dont think each emulator launches the same way, so you may want to search for the code to launch whatever emulator you choose from command line, and put that into the rc.local file.
This will bypass emulation station going straight to the emulator. If i press esc on my keyboard (or the equivalent mapped button press for controllers) I can still get back to emulation station (if I really want to). That way I can still take advantage of the awesomeness of ES and all the ROM/controller/button configurations, or F4 to access command line.
If you want to launch to a single game, I think you just have to list the ROM file aafter the emulator command:
sudo /opt/retropie/emulators/mame4all-pi/mame ROMFILENAME
.
