UART with fopen

Well, the stdio functions aren't in the small library - removing them is most of what is done to make it 'small'.

Possibly you can use the Posix open() and write() functions, but they still require a significant amount of interface 'glue' before you get to the device driver.

For a small system you have to stop pretending you are writing a portable application for a large operating system and start writing code that is specific to the hardware you actually have.

The means that you need to directly call the 'driver' functions, and probably change the driver internals to behave the way you need them to behave, instead of relying on 'standard' driver semantics.