Make serial console enable-able for devices

On some devices (e.g. the Pine64 devices), we want a getty running on the serial console, by default (at least in my opinion).

I'd say we introduce a deviceinfo variable to provide the full string to put into the /etc/inittab file, e.g. ttyS0::respawn:/sbin/getty -L ttyS0 115200 vt100 or ttyMT3::respawn:/sbin/getty -L ttyMT3 921600 vt100 (or a "structured one" e.g. ttyS0;115200 as we might not need to have more than the /dev node and the baud rate configurable)