refactor: read the roms root from romfolders

platforms/romfolders/<platform>.yml records the roms root from the OS's
own source (roms_root path and from_bios), the same fact the registry's
roms_directory restated for ROCKNIX alone. The manifest's roms_directory
now carries from_bios, which the installer follows from a BIOS folder of
the platform's name, and the registry key is gone. A list that installs
above its BIOS folder (AmberELEC) holds its roms root in the install
tree: a file a launcher runs from there is a plain dest. emulators_directory
stays in the registry: no rom folders file names emulator folders.

scripts/verify.py

@@ -794,6 +794,8 @@ def find_undeclared_files(
                     entry["members"] = members
                 if f.get("layout_path"):
                     entry["layout_path"] = f["layout_path"]
+                if f.get("unprefixed"):
+                    entry["unprefixed"] = True
                 if f.get("unplaced"):
                     entry["unplaced"] = f["unplaced"]
                 elif f.get("root") in OUTER_ROOTS: