— Update October 2020 —
While this still works, I consider it obsolete and advise against installing it. I noticed that it has no audio over HDMI and an annoying screen tearing / scrolling lag issue that makes this Retropie setup not fun to play.
Instead, I strongly recommend using Lakka for the Banana Pi. It runs a lot smoother and comes with the latest versions of RetroArch and all the available cores. You can download it here: Lakka image for Banana Pi M1 - Libretro Forums
— Original Post —
I made an easy to understand guide how to install RetroPie on the Banana Pi M1. It is based on Armbian 5.90. It shows how to configure the system and build RetroPie yourself.
I also uploaded an SD card image with everything already installed.
The guide and image file are available on my GitHub page: