Please keep in mind this console is more like a Raspberry Pi Zero/Raspberry Pi 2 than a SNES Classic or Super Retro-cade. Since it doesn't affect the base OS or the recovery partition, if it doesn't work for you, or you don't like it, just install the update from the Atgames website again and it will remove all changes. I did not test it extensively, and it may brick the console (as every software update). It removes the file extension filter from the SD Card and include support to load arcade games from the SD Card using Final Burn Alpha or MAME 2000. This CFW doesn't work with the Legends Flashback 2019 (the one with 100/110 games).
I'd like to share my first custom firmware for the Atgames Legends Flashback 2018.