Install RetroPie in Ubuntu Server 16.04 LTS
The official RetroPie’s wiki recommends Ubuntu desktop for x86. The main inconvenience is you got a huge desktop installation just for run EmulationStation (ES) and the emulators.
This post explains how to configure your Ubuntu Server 16.04 LTS for autologin and launching ES. Before to proceed, follow the installation steps in RetroPie’s wiki section 2.1.
Install the required packages for the X environment and the lightweight window manager TWM
Allow the autologin for the user pi in Ubuntu (I assumed that he is your main user)
And finally run as pi user
Reboot and enjoy!