Revert "Remove double-backslashes from grub conf file".

This reverts commit b861bf8d, because according to @mdorman [1] this
change rendered his NixOS systems unbootable, and we probably don't want that.

[1] https://github.com/NixOS/nixpkgs/commit/b861bf8ddff677031e93a8e1c55311be63e0eb63#commitcomment-16058598