py: Add execfile function (from Python 2); enable in stmhal port.

Adds just 60 bytes to stmhal binary.  Addresses issue #362.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 880e469..1700332 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -340,6 +340,11 @@
 #define MICROPY_PY_BUILTINS_COMPILE (0)
 #endif
 
+// Whether to support the Python 2 execfile function
+#ifndef MICROPY_PY_BUILTINS_EXECFILE
+#define MICROPY_PY_BUILTINS_EXECFILE (0)
+#endif
+
 // Whether to set __file__ for imported modules
 #ifndef MICROPY_PY___FILE__
 #define MICROPY_PY___FILE__ (1)