fix base64 encoding of key field in config

The security.privateKey and security.publicKey fields are of type bytes,
but the protobuf MessageToDict converts them to base64 encoded strings.
When importing the config again, this is read as a string, which breaks
the import. Instead, the value needs to be prefixed with "base64:", so
the type handling logic on import kicks in and decodes the value to a
bytes array again.

Fixes: #678

Author: fmoessbauer

Diff for: meshtastic/__main__.py

@@ -1034,6 +1034,12 @@ def export_config(interface):
                 prefs[meshtastic.util.snake_to_camel(pref)] = config[pref]
             else:
                 prefs[pref] = config[pref]
+            # mark base64 encoded fields as such
+            if pref == "security":
+                if 'privateKey' in prefs[pref]:
+                    prefs[pref]['privateKey'] = 'base64:' + prefs[pref]['privateKey']
+                if 'publicKey' in prefs[pref]:
+                    prefs[pref]['publicKey'] = 'base64:' + prefs[pref]['publicKey']
         if mt_config.camel_case:
             configObj["config"] = config
         else: