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>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index e671a2e..ec3b22e 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -160,6 +160,9 @@
         self.align_end = fdt_util.GetInt(self._node, 'align-end')
         self.offset_unset = fdt_util.GetBool(self._node, 'offset-unset')
 
+    def GetDefaultFilename(self):
+        return None
+
     def AddMissingProperties(self):
         """Add new properties to the device tree as needed for this entry"""
         for prop in ['offset', 'size', 'image-pos']: