— 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. I am currently working on installing Lakka on the Banana Pi. The older Lakka 2.1-rc5 with 3.4x Kernel already works, and I had some success compiling and installing the newest Lakka version that has not been released yet. It works with the very latest linux kernels (5.x) and contains much newer drivers for the Mali GPU that allow for a much smoother experience. See my posts down below for downloadable Lakka images.
— 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: