Re-enable interrupts before directly enqueuing characters in the UART FIFO.

Not sure why, but this reduces the occurrence rate of an occasional ~3.25 or
~7μs intercharacter delay, which was interfering with use of the UART to
generate precise timing pulses (such as driving WS2812 LEDs).

Author: alltheblinkythings

cores/esp8266/HardwareSerial.cpp

@@ -627,12 +627,13 @@ size_t HardwareSerial::write(uint8_t c) {
         return 0;
     _written = true;
 
+    bool tx_now = false;
     while(true) {
         {
             InterruptLock il;
             if(_tx_buffer->empty()) {
                 if(uart_get_tx_fifo_room(_uart) > 0) {
-                    uart_transmit_char(_uart, c);
+                    tx_now = true;
                 } else {
                     _tx_buffer->write(c);
                     uart_arm_tx_interrupt(_uart);
@@ -644,6 +645,9 @@ size_t HardwareSerial::write(uint8_t c) {
         }
         yield();
     }
+    if (tx_now) {
+        uart_transmit_char(_uart, c);
+    }
     return 1;
 }