[Feature Request] Allow enabling hardware flow control for USART

[jg2562]

As far as I could tell, I could not find any hardware flow control options for the usarts. It looks like the data structures in place, so it would just need to be added to the config.

I did see however a comment mentioning hardware flow control, does that mean there's a reason that it doesn't work?

I also did some testing on my Nucleo dev kit of enabling hardware flow control and using the usart attached to the ST-LINK/ micro USB, but it didn't seem to prevent data loss, so I'm not sure if it's possible to enable.