Implement the Wire.flush() funtion

[watershade]

1. implement flush of wire. (flush functions have never been implemented before. But it is useful.)

[watershade]

Wire.flush() is useful, if I haven't read obsolete data from it. I will get a wrong number via Wire.available() because the size have include the obsolete data. And If I use Wire.read(), the data I read out is include the obsolete data also. Yes, I can read the obsolete data out via Wire.read(), but it is not necessary. If Wire.flush() is I implemented, It will become perfect.
Thanks！

[watershade]

@facchinm Hi Martino, I don't know if you time to check this merge. Thanks.

[tokiedokie]

Duplicate of #760

[pennam]

Flush should be intended as a way to send all data out of the TX buffer not to cleanup the RX. See Serial.flush() https://www.arduino.cc/reference/it/language/functions/communication/serial/flush/

[tokiedokie]

Should the RX buffer cleanup API be called clear()?

[pennam]

Should the RX buffer cleanup API be called clear()?

I don't think there is a standard Arduino API, but personally i think is a more appropriate name.

[watershade]

@pennam Thanks for your reply.

I remember flush really flush all data away in old Arduino version.

Prior to Arduino 1.0, this instead removed any buffered incoming serial data.

I will check the new version and try to find a better name. Maybe clear and it is just like @tokiedokie have said .

Thanks again.