filesystem: add placeholder export method in CardFile base class

We add export methods in subclasses of CardFile but the base class
itself lacks an export method. To make the code more readable and
to avoid unnecessary exceptions, les's add a default export method
that just returns a comment.

Related: OS#6092
Change-Id: Ife2a9bad14750db84a87fab907297028c33f1f7d

pySim/filesystem.py

@@ -298,6 +298,17 @@ class CardFile:
             return True
         raise ValueError("self.service must be either int or list or tuple")
 
+    @staticmethod
+    def export(as_json: bool, lchan):
+        """
+        Export file contents in the form of commandline script. This method is meant to be overloaded by a subclass in
+        case any exportable contents are present. The generated script may contain multiple command lines separated by
+        line breaks ("\n"), where the last commandline shall have no line break at the end
+        (e.g. "update_record 1 112233\nupdate_record 1 445566"). Naturally this export method will always refer to the
+        currently selected file of the presented lchan.
+        """
+        return "# %s has no exportable contents" % str(lchan.selected_file)
+
 
 class CardDF(CardFile):
     """DF (Dedicated File) in the smart card filesystem.  Those are basically sub-directories."""