The world of emulation and Raspberry Pi are linked from the beginning. One of the most visited post in this blog is the HowTo play emulators on OpenELEC, which will try to update the cores soon.
Meanwhile, I bring an alternative: An image already prepared to do everything easier, and one more thing...
Follow the instructions here.
You can update the cores thanks to the user Nasp (07/27/15). Just uncompress the link below (MEGA) on /storage/.kodi/userdata/addon_data/emulator.tools.retroarch/system/bin.
Updated cores: mega.co.nz > emulator.tools.retroarch-1.0.3.zip