I've had my Recalbox running on a rPi3+ for a few days. I like it so much, I'm building one for my brother and step-son. I'm willing to bet someone else has already noted this, but I could not find it via a Google Search and it was incredibly frustrating. As such, I'm hoping that this helps someone else out in the future. I also did a search on the forum before posting, maybe I'm thinking too hard and that's why I didn't find it.
I wanted to burn through Doom one more time now that I got my Recalbox up and running, but I didn't want to take the time to "Get Gud" like I did in 1996. I'm thinking this is like a great movie and you just want to fast forward through the boring set-up and find the most entertaining parts since you already know the backstory.
So I wanted to get some cheat codes working in PrBoom but for the life of me, could not get them to "stick" (and it didn't help that the files I found online are crap). As soon as I exit RetroArch, they were gone. I could not download them via online updater.
I even tried creating a them in RA and "Save Cheat File As" through the menu, but I don't know where the file goes, if it's saved at all. "find / -name FILENAME_I_JUST_CREATED.cht - print" yields no results.
Here's how I did it.
Log into your Recalbox via SSH. Each line is typed individually.
mount -o remount, rw /
cp DOS/DOOM.cht Doom/Doom.cht
First line remounts the partition as a read-write, the system won't let you make changes otherwise.
second line changes the directory to the actual cheat file location. I'm pretty sure "custom" that you find in the load cheat file menu is a symlink of some sort or hard coded to /recalbox/share/cheats/
third line creates a "Doom" directory which is case sensitive.
fourth line copies the already existing DOS Doom cheat file to the new directory with the case sensitive file name.
I hope this helps someone out because there's plenty of "Doom.cht" files online that are filled with hex codes that don't work. I'm willing to bet most people simply give up.