nixos/grub: Implements use of file format for splashImage.

GRUB 2.0 supports png, jpeg and tga. This will use the image's suffix to
load the right module.

As jpeg module is named jpeg, jpg is renamed jpeg.

If the user uses wrong image suffix for an image, it wouldn't work anyway.

This will leave up to two additional left-over files in /boot/ if user switches
through all the supported file formats. The module already left the png
image if the user disabled the splash image.