Here is the custom firmware for the EU version (Mega Drive Flashback HD 2019 / Mega Drive Flashback HD 2018):
If you are looking for the files to the US version (Genesis Flashback HD 2018), look at
this post or download these files:
Update for Genesis Flashback HD (US) with Sega CD support (Genesis Plus GX)
Update for Genesis Flashback HD (US) with 32x support (Picodrive)
I will update my other post adding these links. Thanks
@dcuk7 for kindly provide the dump from his Mega Drive Flashback. I've tested with my Genesis Flashback 2018 and it was properly converted to a "Mega Drive Flashback" so it should work in all versions (UK, Germany, Australia).
Just as a recap, this custom firmware unlocks with file extensions in the GAME folder, so it will read all files in this folder. This mod enable Genesis Plus GX to load Master System, Game Gear and Sega CD games from the SD Card.
The "Picodrive" update has the same mod but replaces Genesis Plus GX with Picodrive. This update add support to 32x and fixed the speed and sound issues with PAL games (I've tested with "Sonic 2" for Master System), but it breaks Game Gear (Picodrive doesn't support GG) and Sega CD (I couldn't find if there is a specific folder to drop the BIOS files).
Regarding the 'technical aspects', Genesis Flashback HD and Mega Drive Flashback HD are exactly the same device, using the same emulator and they have the same 'rootfs' partition. The only changes between Genesis and Mega Drive is the box art in the "roms" partition and a few images in the emulator partition.