uart(4) - make the 8250 uart baudrate tolerance build time tweakable.

It turns out on a 16550 w/ a 25MHz SoC reference clock you get a little
over 3% error at 115200 baud, which causes this to fail.

Just .. cope. Things cope these days.

Default to 30 (3.0%) as before, but allow UART_DEV_TOLERANCE_PCT to be
set at build time to change that.