Add default_folder config setting

[wins1ey]

Set the default folder in cubeboot.ini with the key default_folder. Example:

default_folder = games

Cubeboot will start with that folder open instead of /.

All of these opened the games folder on boot correctly:

default_folder = games
default_folder = games/
default_folder = /games
default_folder = /games/

Recreated this PR upstream here from https://github.com/makeo/cubiboot/pull/2