Here is a rough overview of what i did to get it al working, some steps are more detailed than others, i will try to share as much as i can.

1. Download the porta pi image (google it), this is a pre-configured pack of retropie and emulationstation and has a bunch of emulators installed.

2. Mount it on a SD card using Win32 Disk Imager.

3. Edit the GPIO mapping (See step 2).

4. Place your roms on a USB stick and transfer them to your Raspberry Pi and place them in the correct folders inside /home/pi/RetroPie/roms (for examle, place your GBA games inside /home/pi/RetroPie/roms/GBA).

5. For the GBA emulator to work you need to download a BIOS and place it on your pi, see step 3 for detailed instructions.

6. Getting the startup splash screen to work (see step 4).

Edit GPIO mapping :

1. Open retrogame config file :

CD /home/pi/Retrogame

sudo nano retrogame.c

2. Scroll down until you see the input array.

3. For a 1-player arcade machine you need to remove the player 2 inputs for it to work.

4. Edit the GPIO pin numbers for player 1 arcording to your wiring.

5. Press CTRL + O to save your changes

6. Press CTRL + X to exit the editor

7. Make the file and reboot :

make

sudo reboot

Getting the GBA emulator to work :

1. Download gba_bios.bin.

2. Transfer gba_bios.bin to /home/pi/retropie/BIOS from a USB stick.

3. Reboot your pi :

sudo reboot

4. Move the file to it's destination folder :

cd /home/pi/RetroPie/BIOS

sudo cp gba_bios.bin /opt/retropie/emulators/gpsp/raspberrypi

5. Start emulationstation :

emulationstation

6. Configuring input controls for the GBA emulator :

Open any game.

Press F10 while in the game.

Choose the 'input keyboard' option.

Here you can change your keyboard inputs.

Installing a splash screen that you will see at startup instead of a list of commands.

This kindof works, but only for the last part of the loading proces, i haven't found a better way yet but this is better than nothing.

First of all, you need to install fbi:

apt-get install fbi

Open the 'asplashscreen' file :

CD /etc/init.d/
sudo nano asplashscreen

#! /bin/sh
### BEGIN INIT INFO
# Provides: asplashscreen
# Required-Start:
# Required-Stop:
# Should-Start:
# Default-Start: S
# Default-Stop:
# Short-Description: Show custom splashscreen
# Description: Show custom splashscreen
### END INIT INFO
do_start () {
/usr/bin/fbi -T 1 -noverbose -a /etc/splash.png
exit 0
}
case "$1" in
start|"")
do_start
;;
restart|reload|force-reload)
echo "Error: argument '$1' not supported" >&2
exit 3
;;
stop)
# No-op
;;
status)
exit 0
;;
*)
echo "Usage: asplashscreen [start|stop]" >&2
exit 3
;;
esac
:

Then make that script executable and install it for init mode rcS:

sudo chmod a+x /etc/init.d/asplashscreen
sudo insserv /etc/init.d/asplashscreen
Reboot and watch your custom splash screen:
sudo reboot