system: Make /boot/grub/grub.cfg an indirect GC root.

Fixes <http://bugs.gnu.org/19160>.

* guix/scripts/system.scm (install-grub*): Make /boot/grub/grub.cfg an
  indirect GC root.
* gnu/build/install.scm (install-grub): Make TARGET a symlink.
* gnu/build/vm.scm (register-grub.cfg-root): New procedure.
  (initialize-hard-disk): Use it.