bootloader: Use <menu-entry> for the bootloader side.

bootloader: Use <menu-entry> for the bootloader side.

* gnu/bootloader.scm (menu-entry-device-mount-point): New variable.  Export it.
(<menu-entry>: New field "device".
* gnu/bootloader/grub.scm (grub-confgiuration-file): Handle <menu-entry>
entries.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Handle
<menu-entry> entries.
* gnu/system.scm (menu->entry->boot-parameters): Delete variable.
(boot-parameters->menu-entry): New variable.  Export it.
(operating-system-bootcfg): Make OLD-ENTRIES a list of <menu-entry>.
* guix/script/system.scm (reinstall-bootloader): Fix bootcfg usage.
(perform-action): Fix bootcfg usage.