Support the ReadReady and WriteReady traits on UART

[nils-van-zuijlen]

The ReadReady and WriteReady traits make generic checking of UART buffers possible.

As there already exist a uart_is_readable and uart_is_writable methods, I would hope it would be possible to implement them.

I can make a PR if you want.

[jannic]

Sounds like a good idea, and a PR would of course be welcome.

[Sympatron]

This was done in #837 and #841.