PPSSPP core can't load assets

Playstation Portable core [requires](https://docs.libretro.com/library/ppsspp/#bios) original PPSSPP assets for displaying PSP keyboard and correct save files management.

I copied all required assets to the `~/.var/app/org.libretro.RetroArch/config/retroarch/system/PPSSPP` directory, but it didn't work.


Same happens with the dolphin-emu core for the GameCube.

Start with the linked PPSSPP BIOS documentation and the reported Flatpak system path, then compare how the PPSSPP and dolphin-emu cores locate their required assets. Done means the copied assets are recognized by the cores, enabling the PSP keyboard and correct save-file management, with the same behavior checked for dolphin-emu.

Written by the indexing model from the issue text.