Since 0.4.9: Available on Serial1, Serial2, etc..
Since 0.5.0: Available on USB Serial (Serial)
Since 0.6.0: Available on
Retrieves the number of bytes (characters) that can be written to this serial port without blocking.
blockOnOverrun(false) has been called, the method returns the number of bytes that can be written to the buffer without causing buffer overrun, which would cause old data to be discarded and overwritten.