binman: Add an entry method for getting the default filename

Various entry implementations provide a way to obtain the default filename
for an entry. But at present there is no base-class implementation for
this function. Add one so that the API is defined.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/entry_test.py

@@ -65,6 +65,11 @@ class TestEntry(unittest.TestCase):
         sub_entry = entry.Entry(None, None, sub_node, read_node=False)
         self.assertEqual('root.subnode', sub_entry.GetUniqueName())
 
+    def testGetDefaultFilename(self):
+        """Trivial test for this base class function"""
+        import entry
+        base_entry = entry.Entry(None, None, None, read_node=False)
+        self.assertIsNone(base_entry.GetDefaultFilename())
 
 if __name__ == "__main__":
     unittest.main()