Add half-duplex support to HardwareSerial.

[ghent360]

Summary
TMC2209 and 2208 drivers use UART communication with a single wire. Enabling half-duplex support in the HardwareSerial class would enable using these drivers without wasting another pin for the RX signal.

Proposed API is to enable half-duplex mode if the RX pin on the serial port is NC. Also added a method to set the half-duplex mode explicitly.

Validation

• Ensure Travis CI build is passed.
• Demonstrate the code is solid. [e.g. Provide a sketch]
  Tested using:
  https://gist.github.com/ghent360/dc2a7226919af522e4a5914da1ad5252

[fpistm]

Hi @ghent360
Thanks for the PR.
Some minor remarks.

[AnHardt]

Proposed API is to enable half-duplex mode if the RX pin on the serial port is NC. Also added a method to set the half-duplex mode explicitly.

Could also be nice if TX_pin == RX_pin would set up half-duplex.

[fpistm]

Proposed API is to enable half-duplex mode if the RX pin on the serial port is NC. Also added a method to set the half-duplex mode explicitly.

Could also be nice if TX_pin == RX_pin would set up half-duplex.

With a SoftwareSerial this could be possible but with HardwareSerial there no U(S)ART RX and TX on the same pin, as far as I know.

[fpistm]

Thanks @ghent360 , I have to test it. Do you have a sketch example to test this? Else I will write one.

[fpistm]

Proposed API is to enable half-duplex mode if the RX pin on the serial port is NC. Also added a method to set the half-duplex mode explicitly.

Could also be nice if TX_pin == RX_pin would set up half-duplex.

With a SoftwareSerial this could be possible but with HardwareSerial there no U(S)ART RX and TX on the same pin, as far as I know.

My bad, after some read some documentations this would be possible.
I'm currently reviewing this PR and it seems there is an issue on how the Tx pin is configured.
Documentation recommend this:

The USART can be configured to follow a single-wire half-duplex protocol where the Tx and Rx lines are internally connected. In this communication mode, only the Tx pin is used forboth transmission and reception.
The Tx pin is always released when no data is transmitted, thus, it acts as a standard I/O in idle or
reception modes.
This means that the I/O must be configured so that the Tx pin is configured as an alternate function open-drain with an external pull-up.

Currently this is not the case as by default it is set in PushPull

[fpistm]

I will provide a PR to extend how enable half-duplex.
And a small fix when Serial is disabled.