Move <boot-parameters> to (gnu system).

* guix/scripts/system.scm (previous-grub-entries)
  (display-system-generation): Use accessors instead of matching
  <boot-parameters>.
  (boot-parameters, boot-parameters?, boot-parameters-label)
  (boot-parameters-root-device, boot-parameters-kernel)
  (boot-parameters-kernel-arguments, read-boot-parameters): Move to...
* gnu/system.scm: ... here. Export them.