[RF] Change ESP8266 default RF_RECEIVER_GPIO from 0 to 4 (#2240) (#2323)

GPIO0 (D3) is the ESP8266 boot strapping pin: an RF receiver holding the
data line low at reset forces the board into flash mode, so a stock
nodemcuv2-rf build never starts after a restart with the receiver wired
to the documented default pin. Default to GPIO4 (D2) instead, which the
docs already recommend as the workaround. Environments that need other
pins (srfb-direct, sonoff-rfr3, rf-wifi-gateway) already override the
value explicitly and are unaffected.

main/config_RF.h

@@ -230,7 +230,7 @@ extern RFConfiguration iRFConfig;
 /*-------------------PIN DEFINITIONS----------------------*/
 #ifndef RF_RECEIVER_GPIO
 #  ifdef ESP8266
-#    define RF_RECEIVER_GPIO 0 // D3 on nodemcu // put 4 with rf bridge direct mod
+#    define RF_RECEIVER_GPIO 4 // D2 on nodemcu, don't use 0 (D3): a receiver holding it low at reset forces the ESP8266 into flash mode
 #  elif defined(ESP32)
 #    define RF_RECEIVER_GPIO 27 // D27 on DOIT ESP32
 #  endif