— Update October 2020 —
While this still works, I consider it obsolete and advise against installing it. I noticed 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.
Scripts for installing Retropie on Banana Pi M1 (Allwinner A20)
Here are the instructions how to configure the system and build RetroPie for yourself: https://github.com/bernhard263/Bananapi-Retropie/wiki
Here is a ready-made SD card image for the Banana Pi that has everything already installed: https://github.com/bernhard263/Bananapi-Retropie/wiki/Image
大概跟我現在用新版的Armbian(Debian系統)+Kernel 5.8.11跑Banana Pi M1，RestroPie好不容易搞定(但是要先登入進X Window才能執行Emulationstation)。