esp8266/esp_mphal: Add ets_esf_free_bufs(), etc. functions.

Returning free number of various WiFi driver packet buffers.
diff --git a/esp8266/esp_mphal.h b/esp8266/esp_mphal.h
index 2de5fcc..13b1c8f 100644
--- a/esp8266/esp_mphal.h
+++ b/esp8266/esp_mphal.h
@@ -77,4 +77,7 @@
 #define mp_hal_pin_read(p) pin_get(p)
 #define mp_hal_pin_write(p, v) pin_set((p), (v))
 
+void *ets_get_esf_buf_ctlblk(void);
+int ets_esf_free_bufs(int idx);
+
 #endif // _INCLUDED_MPHAL_H_