I made a shell script that will update retroarch to the latest version, install filters etc. You can copy what you need from here: https://github.com/hpcodecraft/gameshell-setup/blob/master/retroarch/setup_retroarch.sh
Update: You can also run it by a setup script I wrote now, which can also do other things like resize your filesystem to fill the SD card or install an alias to take screenshots via SSH. You can check it out in the same repo, the retroarch setup is a part of it: https://github.com/hpcodecraft/gameshell-setup