extmod/modlwip: slip: Use stream protocol and be port-independent.
Based on the original patch by Galen Hazelwood:
https://github.com/micropython/micropython/pull/1517 .
diff --git a/py/objmodule.c b/py/objmodule.c
index 63cccde..02a106d 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -189,6 +189,9 @@
#if MICROPY_PY_USSL
{ MP_OBJ_NEW_QSTR(MP_QSTR_ussl), (mp_obj_t)&mp_module_ussl },
#endif
+#if MICROPY_PY_LWIP
+ { MP_OBJ_NEW_QSTR(MP_QSTR_lwip), (mp_obj_t)&mp_module_lwip },
+#endif
// extra builtin modules as defined by a port
MICROPY_PORT_BUILTIN_MODULES