Baud rate selection on serial binding as parameter

The serial binding defaults the baud rate to 9600.

These changes allow setting the baud rate as a parameter with the item configuration.

Example from demo.items:

String Arduino				"XXXXTemperature" { serial="/dev/ttyACM0@115200" }

This will initialize the serial port /dev/ttyACM0 with a baud rate of 115200.

Backward compability is given, as if no baud rate is specified, the serial binding defaults to 9600 bauds.

Something like this will init the serial port /dev/ttyACM0 with 9600 baud.
String Arduino				"XXXXTemperature" { serial="/dev/ttyACM0" }