system: Rename operating-system-grub.cfg to operating-system-bootcfg.

* gnu/system.scm (operating-system-grub.cfg): Rename to...
(operating-system-bootcfg): ... this.
* gnu/system/vm.scm (system-disk-image): Use operating-system-bootcfg.
(system-qemu-image): Use operating-system-bootcfg.
(system-qemu-image/shared-store): Use operating-system-bootcfg.
* guix/scripts/system.scm (perform-action): Use operating-system-bootcfg.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>