install: Enable SSH in installation image.

* gnu/system/install.scm (%installation-services): Add OPENSSH-SERVICE-TYPE.
* doc/guix.texi (Preparing for Installation)[Networking]: Document it.

doc/guix.texi

@@ -36,7 +36,8 @@ Copyright @copyright{} 2017 Federico Beffa@*
 Copyright @copyright{} 2017 Carlo Zancanaro@*
 Copyright @copyright{} 2017 Thomas Danckaert@*
 Copyright @copyright{} 2017 humanitiesNerd@*
-Copyright @copyright{} 2017 Christopher Allan Webber
+Copyright @copyright{} 2017 Christopher Allan Webber@*
+Copyright @copyright{} 2017 Marius Bakke
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -7446,6 +7447,17 @@ ping -c 3 gnu.org
 Setting up network access is almost always a requirement because the
 image does not contain all the software and tools that may be needed.
 
+@cindex installing over SSH
+If you want to, you can continue the installation remotely by starting
+an SSH server:
+
+@example
+herd start ssh-daemon
+@end example
+
+Make sure to either set a password with @command{passwd}, or configure
+OpenSSH public key authentication before logging in.
+
 @subsubsection Disk Partitioning
 
 Unless this has already been done, the next step is to partition, and

gnu/system/install.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2016 Andreas Enge <andreas@enge.fr>
+;;; Copyright © 2017 Marius Bakke <mbakke@fastmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -25,6 +26,7 @@
   #:use-module (guix monads)
   #:use-module ((guix store) #:select (%store-prefix))
   #:use-module (gnu services shepherd)
+  #:use-module (gnu services ssh)
   #:use-module (gnu packages admin)
   #:use-module (gnu packages bash)
   #:use-module (gnu packages bootloaders)
@@ -262,6 +264,16 @@ You have been warned.  Thanks for being so brave.
           ;; To facilitate copy/paste.
           (gpm-service)
 
+          ;; Add an SSH server to facilitate remote installs.
+          (service openssh-service-type
+                   (openssh-configuration
+                    (port-number 22)
+                    (permit-root-login #t)
+                    ;; The root account is passwordless, so make sure
+                    ;; a password is set before allowing logins.
+                    (allow-empty-passwords? #f)
+                    (password-authentication? #t)))
+
           ;; Since this is running on a USB stick with a unionfs as the root
           ;; file system, use an appropriate cache configuration.
           (nscd-service (nscd-configuration