fix: say which cores find their BIOS by content

DuckStation (bios.cpp:208, 397-427), SwanStation (host_interface.cpp:
136-194), rustation (lib.rs:107-160), FS-UAE (rommgr.cpp:92-151),
ares (firmware.cpp:116-136) and Altirra (uifirmwarescan.cpp:29-113)
scan a folder and identify firmware by hash; Ymir does in its
standalone build only. FS-UAE still opens rom.key by name
(rommgr.cpp:1397-1498). The export then stops writing their labels
into platform lists: Batocera's psx block goes from 106 entries to 10,
and System.dat's scph5000.bin keeps its own hashes.

emulators/ymir.yml

@@ -15,6 +15,7 @@ core_version:
 display_name: "Sega - Saturn (Ymir)"
 cores:
   - ymir
+found_by: {standalone: content}
 systems:
   - sega-saturn