esp8266: Add esp.socket class, with ESP-style socket functionality.

 * UDP currently not supported
 * As there is no way (that I know of) the espconn_regist_connectcb()
   callback can recognize on which socket has the connection arrived,
   only one listening function at a time is supported
diff --git a/esp8266/mpconfigport.h b/esp8266/mpconfigport.h
index dcd9407..cae5398 100644
--- a/esp8266/mpconfigport.h
+++ b/esp8266/mpconfigport.h
@@ -71,7 +71,11 @@
 #define MP_STATE_PORT MP_STATE_VM
 
 #define MICROPY_PORT_ROOT_POINTERS \
-    const char *readline_hist[8];
+    const char *readline_hist[8]; \
+    \
+    /* Singleton instance of scan callback, meaning that there can
+       be only one concurrent AP scan. */ \
+    mp_obj_t scan_cb_obj; \
 
 // We need to provide a declaration/definition of alloca()
 #include <alloca.h>