device-nokia-n900: Increase simple terminal font size to 12

The stock size of the font in simple terminal was barely readable in i3wm.

I increased the font size by passing the -f "Liberation Mono-12" to i3-sensible-terminal
in i3cwm.conf. The parameter gets forwarded to st terminal.

If you think this could be done in a better way let me know!

Here is how it looked before the change

And here is how it is after