py/objstr: Make .partition()/.rpartition() methods configurable.

Default is disabled, enabled for unix port. Saves 600 bytes on x86.
diff --git a/py/objstrunicode.c b/py/objstrunicode.c
index 495ef3a..7b50dfe 100644
--- a/py/objstrunicode.c
+++ b/py/objstrunicode.c
@@ -246,8 +246,10 @@
     { MP_ROM_QSTR(MP_QSTR_format), MP_ROM_PTR(&str_format_obj) },
     { MP_ROM_QSTR(MP_QSTR_replace), MP_ROM_PTR(&str_replace_obj) },
     { MP_ROM_QSTR(MP_QSTR_count), MP_ROM_PTR(&str_count_obj) },
+    #if MICROPY_PY_BUILTINS_STR_PARTITION
     { MP_ROM_QSTR(MP_QSTR_partition), MP_ROM_PTR(&str_partition_obj) },
     { MP_ROM_QSTR(MP_QSTR_rpartition), MP_ROM_PTR(&str_rpartition_obj) },
+    #endif
 #if MICROPY_PY_BUILTINS_STR_CENTER
     { MP_ROM_QSTR(MP_QSTR_center), MP_ROM_PTR(&str_center_obj) },
 #endif