Rename configuration variables controling Python features.

Now of the form MICROPY_PY_*.  See issue #35.
diff --git a/py/builtinimport.c b/py/builtinimport.c
index d2b1870..fa1a839 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -73,7 +73,7 @@
     // extract the list of paths
     uint path_num = 0;
     mp_obj_t *path_items;
-#if MICROPY_ENABLE_MOD_SYS
+#if MICROPY_PY_SYS
     mp_obj_list_get(mp_sys_path, &path_num, &path_items);
 #endif