serial_open: Take uint32_t instead of unsigned int

Some platforms have 16 bit ints, which is not enough to hold values like
115200.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Ported serial code to Arduino

Only tested on the Arduino Mega 2560 so far, but I expect it will work on most
boards.  I didn't read the specification, but based the logic on
HardwareSerial.cpp.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>