fix: read recalbox mandatory per named core

An element lists its cores. Its mandatory flag now follows whether those
cores need the file, and stays as written where none of them reads it.
cdi/cdibios.zip keeps mandatory=false (same_cdi does not read it), the
two amiberry kickstarts keep theirs, saturn_bios.bin becomes optional
for the three HLE cores it names.

scripts/exporter/base_exporter.py

@@ -200,6 +200,14 @@ class BaseExporter(ABC):
             return self.single_value(fe, field_name)
         return ",".join(fe.hashes(field_name))
 
+    def required_value(self, fe: NativeFile) -> bool:
+        """The requirement the render writes for an entry.
+
+        The model's flag is true when any core needs the file. A format
+        that names the cores of each entry narrows it to those cores.
+        """
+        return fe.required
+
     def states(self, fe: NativeFile, field_name: str) -> bool:
         """Whether the written file carries this entry's corrected field.