This commit adds a `baud` argument for Serial and RawSerial objects:
- there is a new `Serial` constructor with a mandatory baud rate
argument. The old constructor also got a `baud` argument with a
default value (to keep backward compatibility).
- the `RawSerial` constructor also got a `baud` argument with a default
value.
There's also a new configuration parameter (`default-serial-baud-rate`)
that can be used to specify the default value of the above `baud`
arguments.
