Weak references for more options to control low speed pins

src/hal/hal_esp32.c

@@ -152,6 +152,7 @@ void init_io(void)
     ESP_LOGI(TAG, "IO initialized");
 }
 
+__attribute__((weak)) // duplicate this symbol if your pin is controlled by e.g. I2C
 void hal_pin_rxtx(u1_t val)
 {
     if (pin_rx_tx == LMIC_UNUSED_PIN)
@@ -160,6 +161,7 @@ void hal_pin_rxtx(u1_t val)
     gpio_set_level(pin_rx_tx, val);
 }
 
+__attribute__((weak)) // duplicate this symbol if your pin is controlled by e.g. I2C
 void hal_pin_rst(u1_t val)
 {
     if (pin_rst == LMIC_UNUSED_PIN)
@@ -252,6 +254,7 @@ void hal_spi_read(u1_t cmd, u1_t *buf, size_t len)
     ESP_ERROR_CHECK(err);
 }
 
+
 void IRAM_ATTR assert_nss(spi_transaction_t* trans)
 {
     gpio_set_level(pin_nss, 0);