This commit is a fixup for a regression introduced by
0bdba6c99b.
Before the regression, it was possible to build images without grub or a
kernel (e.g. to boot other kernels with qemu -kernel.
After the regression, such images fail to build. Since
config.boog.loader.grub.enable is false in that scenario, grub.device is
emptystring. While this happens not to be an issue of `ln`, `dirname`
fails on emptystring.
With this change, we guard both commands to only be run when grub is
actually enabled. Images with and without grub succesfully build with
this change.
d7740c24cb