bug#26339: "extlinux", "extlinux" gpt, bootloader-configuration without package nor installer

[Danny Milosavljevic <dannym@scratchpost.org>]

A possible gnu/bootloader/u-boot.scm:

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2017 David Craven <david@craven.ch>
;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
;;; Copyright © 2017 Danny Milosavljevic <dannym@scratchpost.org>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (gnu bootloader u-boot)
  #:use-module (gnu bootloader extlinux)
  #:use-module (gnu bootloader)
  #:use-module (gnu system)
  #:use-module (gnu packages bootloaders)
  #:use-module (guix gexp)
  #:use-module (guix monads)
  #:use-module (guix records)
  #:use-module (guix utils)
  #:export (u-boot-bootloader))

;;;
;;; Installation procedures.
;;;

(define install-u-boot
  #~(lambda (bootloader device mount-point)
      (if bootloader
        (let ((u-boot-installer (string-append bootloader "/sbin/install-u-boot")) ; doesn't actually exist yet.
              (installation-dir (string-append mount-point "/boot")))
          (unless (zero? (system* u-boot-installer
                                  (string-append "--installdir=" installation-dir)
                                  (string-append "--device" device))
            (error "failed to install U-Boot")))))))

;;;
;;; Bootloader definitions.
;;;

(define u-boot-bootloader
  (bootloader
   (inherit extlinux-bootloader)
   (package #f)
   (name 'u-boot)
   (installer install-u-boot)
   (configuration-file "/boot/extlinux.conf")))