Modified error formatting to remove unicode character.

jsonschema's error.message returns either u' or ' in strings depending on the python version.

tools/config/__init__.py

@@ -393,7 +393,8 @@ class Config(object):
             return self.format_validation_error(error.context[0], path)
         else:
             return "in {} element {}: {}".format(
-                path, str(".".join(str(p) for p in error.absolute_path)), error.message)
+                path, ".".join(p for p in error.absolute_path),
+                error.message.replace('u\'','\''))
 
     def __init__(self, tgt, top_level_dirs=None, app_config=None):
         """Construct a mbed configuration

tools/test/config/invalid_key/test_data.json

@@ -1,5 +1,5 @@
 {
     "K64F": {
-        "exception_msg": "Additional properties are not allowed (u'unknown_key' was unexpected)"
+        "exception_msg": "Additional properties are not allowed ('unknown_key' was unexpected)"
     }
 }

tools/test/config/invalid_key_lib/test_data.json

@@ -1,5 +1,5 @@
 {
     "K64F": {
-        "exception_msg": "Additional properties are not allowed (u'unknown_key' was unexpected)"
+        "exception_msg": "Additional properties are not allowed ('unknown_key' was unexpected)"
     }
 }