esp8266/modnetwork: Add symbolic names for network interfaces: STA_IF & AP_IF.

These are expected to be passed to network.WLAN() to instantiate network
interface objects.
diff --git a/esp8266/modnetwork.c b/esp8266/modnetwork.c
index 40bc459..0391c99 100644
--- a/esp8266/modnetwork.c
+++ b/esp8266/modnetwork.c
@@ -323,6 +323,11 @@
     { MP_OBJ_NEW_QSTR(MP_QSTR_phy_mode), (mp_obj_t)&esp_phy_mode_obj },
 
 #if MODNETWORK_INCLUDE_CONSTANTS
+    { MP_OBJ_NEW_QSTR(MP_QSTR_STA_IF),
+        MP_OBJ_NEW_SMALL_INT(STATION_IF)},
+    { MP_OBJ_NEW_QSTR(MP_QSTR_AP_IF),
+        MP_OBJ_NEW_SMALL_INT(SOFTAP_IF)},
+
     { MP_OBJ_NEW_QSTR(MP_QSTR_STAT_IDLE),
         MP_OBJ_NEW_SMALL_INT(STATION_IDLE)},
     { MP_OBJ_NEW_QSTR(MP_QSTR_STAT_CONNECTING),