Fix MacOS write error caused by unflushed buffer

[jgartrel]

This change is actually a cherry-picked code change from a pull request to shumatech/BOSSA. Special thanks to @kaysievers for this code (see issues: Bossa 1.9.1 occasionally fails mid-download #120 and the subsequent resolution in Pull Request #150)

This change resolved an issue where (I suppose) flash has not been properly erased prior to writing the new code. This results in a verify error and requires code to be re-flashed to the SAMD21.

PosixSerialPort: Call tcdrain() to write serial data

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[jgartrel]

@facchinm, I am hoping that we can create a patch release for this and upload it to the Arduino package repository. What is the process for that?

[jgartrel]

@cmaglie, @per1234 Do you know what the process is for pull requests and patch releases for this repo/project? Could you please point me in the right direction?

[JetForMe]

I had to make this modification locally in order to flash SAM parts. This pull request has been around for ages, it would be nice to get it merged.