Via the standard tx/rx arguments: UART(0, 115200, tx=Pin(15), rx=Pin(13)). Resolves issue #4718.
As per the machine.UART documentation, this is used to set the length of the UART RX buffer.
This is to keep the top-level directory clean, to make it clear what is core and what is a port, and to allow the repository to grow with new ports in a sustainable way.