initialize tchain variable before usage

Author: Olli-Pekka Puolitaival

mbed/mbed.py

@@ -2753,6 +2753,9 @@ def test_(toolchain=None, target=None, compile_list=False, run_list=False,
           app_config=None, test_config=None, coverage=None, make_program=None,
           new=None, generator=None, regex=None, unittests=None, 
           build_data=None, greentea=None, icetea=None):
+
+    tchain = None
+
     # Default behaviour is to run only greentea tests
     if not (greentea or icetea or unittests):
         greentea = True