config: Added missing types to JSON/INI Python reader

Added the missing types EthernetAddr and Current to the JSON/INI file
reader example configs/example/read_config.py.

Also added __str__ to EthernetAddr to make values appear in the same form
in JSON an INI files.

src/python/m5/params.py

@@ -930,6 +930,9 @@ class EthernetAddr(ParamValue):
         from m5.internal.params import EthAddr
         return EthAddr(self.value)
 
+    def __str__(self):
+        return self.value
+
     def ini_str(self):
         return self.value