Write the Uarte implementation

Step-by-Step Solution

add field to the board struct
add struct for the instance, how to figure out what the type of the inner field is
create instance in init, add baudrate, parity etc.
add to instantiation of board struct
impl fmt::Write for the Uarte struct, simple write does not work because of dma
example code with button is not a good idea for the simple button implementation.

I think this is plenty for an hour.