Add support for regular .img files

Supporting .img.xz files is great for our needs, since that's the format we do our images in.

But it wouldn't hurt to also support the uncompressed versions (.img) files.

This has the benefit of being able to flash the images, without doing the compressing step when creating test images.

It also has the added benefit of supporting almost all other ARM images (uncompressed) instead of just Manjaro ARM ones.