Allow custom command to handle the serial console

The need I am addressing with this is making the serial console
auto-login as root. For this right now I'm using the auto-serial-console
program from the linaro-overlay package:

https://launchpad.net/~linaro-maintainers/+archive/overlay/+files/linaro-overlay_1112.2.dsc

In the near future I plan to port the generic parts of that package to
Debian under the name auto-serial-console or some similar