platform: serial: add bytesAvailable() function

platforms/common/Serial.cpp

@@ -69,6 +69,11 @@ bool Serial::close()
 	return _impl.close();
 }
 
+ssize_t Serial::bytesAvailable()
+{
+	return _impl.bytesAvailable();
+}
+
 ssize_t Serial::read(uint8_t *buffer, size_t buffer_size)
 {
 	return _impl.read(buffer, buffer_size);

platforms/common/include/px4_platform_common/Serial.hpp

@@ -61,6 +61,7 @@ public:
 
 	bool close();
 
+	ssize_t bytesAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_ms = 0);