Add SerialEvent() callback to loop processing (#7505)

* Add SerialEvent() callback to loop processing

Match the AVR SerialEvent implicit callback.  Callback is executed
in normal user mode, not IRQ, so standard processing can be uses.

Fixes #752 after 5 years. :)

* Fix style

cores/esp8266/HardwareSerial.cpp

@@ -32,6 +32,14 @@
 #include "HardwareSerial.h"
 #include "Esp.h"
 
+
+// SerialEvent functions are weak, so when the user doesn't define them,
+// the linker just sets their address to 0 (which is checked below).
+// The Serialx_available is just a wrapper around Serialx.available(),
+// but we can refer to it weakly so we don't pull in the entire
+// HardwareSerial instance if the user doesn't also refer to it.
+void serialEvent() __attribute__((weak));
+
 HardwareSerial::HardwareSerial(int uart_nr)
     : _uart_nr(uart_nr), _rx_size(256)
 {}
@@ -162,6 +170,14 @@ size_t HardwareSerial::readBytes(char* buffer, size_t size)
 
 #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL)
 HardwareSerial Serial(UART0);
+
+// Executed at end of loop() processing when > 0 bytes available in the Serial port
+void serialEventRun(void)
+{
+  if (serialEvent && Serial.available()) {
+    serialEvent();
+  }
+}
 #endif
 #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SERIAL1)
 HardwareSerial Serial1(UART1);

cores/esp8266/HardwareSerial.h

@@ -207,4 +207,6 @@ protected:
 extern HardwareSerial Serial;
 extern HardwareSerial Serial1;
 
+extern void serialEventRun(void) __attribute__((weak));
+
 #endif

cores/esp8266/core_esp8266_main.cpp

@@ -195,6 +195,9 @@ static void loop_wrapper() {
     }
     loop();
     loop_end();
+    if (serialEventRun) {
+        serialEventRun();
+    }
     esp_schedule();
 }