nixos/grub: Adds background color and mode options

The background color option is self-explanatory.

The mode is either `normal` or `stretch`, they are as defined by GRUB,
where normal will put the image in the top-left corner of the menu, and
stretch is the default, where it stretches the image without
consideration for the aspect ratio.

 * https://www.gnu.org/software/grub/manual/grub/grub.html#background_005fimage