esp8266/esp_mphal: Provide mp_hal_pin_od_high_dht so DHT works reliably.

The original behaviour of open-drain-high was to use the open-drain mode of
the GPIO pin, and this seems to make driving a DHT more reliable.  See
issue #4233.

ports/esp8266/esp_mphal.h

@@ -91,6 +91,11 @@ void mp_hal_pin_open_drain(mp_hal_pin_obj_t pin);
         if ((p) == 16) { WRITE_PERI_REG(RTC_GPIO_ENABLE, (READ_PERI_REG(RTC_GPIO_ENABLE) & ~1)); } \
         else { gpio_output_set(0, 0, 0, 1 << (p)); /* set as input to avoid glitches */ } \
     } while (0)
+// The DHT driver requires using the open-drain feature of the GPIO to get it to work reliably
+#define mp_hal_pin_od_high_dht(p) do { \
+        if ((p) == 16) { WRITE_PERI_REG(RTC_GPIO_ENABLE, (READ_PERI_REG(RTC_GPIO_ENABLE) & ~1)); } \
+        else { gpio_output_set(1 << (p), 0, 1 << (p), 0); } \
+    } while (0)
 #define mp_hal_pin_read(p) pin_get(p)
 #define mp_hal_pin_write(p, v) pin_set((p), (v))