Rename configuration variables controling Python features.

Now of the form MICROPY_PY_*.  See issue #35.

py/builtinimport.c

@@ -73,7 +73,7 @@ mp_import_stat_t find_file(const char *file_str, uint file_len, vstr_t *dest) {
     // 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