HardwareSerial - availableForWrite() support (#1226)

* HardwareSerial - availableForWrite() support

* fix indentation errors (tab vs space)

cores/esp32/HardwareSerial.cpp

@@ -62,6 +62,10 @@ int HardwareSerial::available(void)
 {
     return uartAvailable(_uart);
 }
+int HardwareSerial::availableForWrite(void)
+{
+    return uartAvailableForWrite(_uart);
+}
 
 int HardwareSerial::peek(void)
 {

cores/esp32/HardwareSerial.h

@@ -40,6 +40,7 @@ public:
     void begin(unsigned long baud, uint32_t config=SERIAL_8N1, int8_t rxPin=-1, int8_t txPin=-1, bool invert=false);
     void end();
     int available(void);
+    int availableForWrite(void);
     int peek(void);
     int read(void);
     void flush(void);

cores/esp32/esp32-hal-uart.c

@@ -248,6 +248,14 @@ uint32_t uartAvailable(uart_t* uart)
     return uxQueueMessagesWaiting(uart->queue);
 }
 
+uint32_t uartAvailableForWrite(uart_t* uart)
+{
+    if(uart == NULL) {
+        return 0;
+    }
+    return 0x7f - uart->dev->status.txfifo_cnt;
+}
+
 uint8_t uartRead(uart_t* uart)
 {
     if(uart == NULL || uart->queue == NULL) {

cores/esp32/esp32-hal-uart.h

@@ -55,6 +55,7 @@ uart_t* uartBegin(uint8_t uart_nr, uint32_t baudrate, uint32_t config, int8_t rx
 void uartEnd(uart_t* uart);
 
 uint32_t uartAvailable(uart_t* uart);
+uint32_t uartAvailableForWrite(uart_t* uart);
 uint8_t uartRead(uart_t* uart);
 uint8_t uartPeek(uart_t* uart);