nova serial log output

[Pirmin Duss]

Hello Alex

After I modified bender to always write `0x3f8` to the BDA (currently 
rather hackish), I'm able to see the serial output.
The `serial_fallback` wasn't considered, as some serial device was found.

Would you be interested in a patch that adds an option to always write 
the value `0x3f8` if it is specified on the console. This in fact would 
be two commits (one for mobro and one for genode).


Best regards, Pirmin