bootloader: Use menu-entry to define custom bootloader entries.

* gnu/bootloader.scm (<menu-entry>): New variable. Export associated getters,
This record is extracted from grub module.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Use
  menu-entry->boot-parameters to convert menu-entry records to
  boot-parameters.
* gnu/bootloader/grub.scm (<menu-entry>): Remove.
(boot-parameters->menu-entry): Remove.
(grub-configuration-file): Use boot-parameters to create configuration
entries.
* gnu/system.scm (menu-entry->boot-parameters): New exported procedure.