system: Add os-with-u-boot.

* gnu/system/install.scm (os-with-u-boot): New procedure.
* gnu/packages/bootloaders.scm (make-u-boot-package): Export.
* doc/guix.texi (Building the Installation Image for ARM Boards): New
subsection.

doc/guix.texi

@@ -8881,6 +8881,22 @@ Have a look at @file{gnu/system/install.scm} in the source tree,
 and see also @ref{Invoking guix system} for more information
 about the installation image.
 
+@subsection Building the Installation Image for ARM Boards
+
+Many ARM boards require a specific variant of the
+@uref{http://www.denx.de/wiki/U-Boot/, U-Boot} bootloader.
+
+If you build a disk image and the bootloader is not available otherwise
+(on another boot drive etc), it's advisable to build an image that
+includes the bootloader, specifically:
+
+@example
+guix system disk-image --system=armhf-linux -e '((@@ (gnu system install) os-with-u-boot) (@@ (gnu system install) installation-os) "A20-OLinuXino-Lime2")'
+@end example
+
+@code{A20-OLinuXino-Lime2} is the name of the board.  If you specify an invalid
+board, a list of possible boards will be printed.
+
 @node System Configuration
 @section System Configuration
 

gnu/packages/bootloaders.scm

@@ -439,7 +439,7 @@ def test_ctrl_c"))
 also initializes the boards (RAM etc).  This package provides its
 board-independent tools.")))
 
-(define (make-u-boot-package board triplet)
+(define-public (make-u-boot-package board triplet)
   "Returns a u-boot package for BOARD cross-compiled for TRIPLET."
   (let ((same-arch? (if (string-prefix? (%current-system)
                                         (gnu-triplet->nix-system triplet))

gnu/system/install.scm

@@ -54,7 +54,8 @@
             novena-installation-os
             pine64-plus-installation-os
             rk3399-puma-installation-os
-            wandboard-installation-os))
+            wandboard-installation-os
+            os-with-u-boot))
 
 ;;; Commentary:
 ;;;
@@ -386,6 +387,19 @@ You have been warned.  Thanks for being so brave.\x1b[0m
                      nvi                          ;:wq!
                      %base-packages))))
 
+(define* (os-with-u-boot os board #:key (bootloader-target "/dev/mmcblk0")
+                         (triplet "arm-linux-gnueabihf"))
+  "Given OS, amend it with the u-boot bootloader for BOARD,
+installed to BOOTLOADER-TARGET (a drive), compiled for TRIPLET.
+
+If you want a serial console, make sure to specify one in your
+operating-system's kernel-arguments (\"console=ttyS0\" or similar)."
+  (operating-system (inherit os)
+    (bootloader (bootloader-configuration
+                 (bootloader (bootloader (inherit u-boot-bootloader)
+                              (package (make-u-boot-package board triplet))))
+                 (target bootloader-target)))))
+
 (define* (embedded-installation-os bootloader bootloader-target tty
                                    #:key (extra-modules '()))
   "Return an installation os for embedded systems.