guix system: Add '--system' option.

* guix/scripts/system.scm (switch-to-system): Add #:system parameter;
  pass it to 'run-with-store'.
  (%options): Add '--system'.
  (guix-system): Pass the 'system' option to 'run-with-store',
  'package-derivation', and 'switch-to-system' calls.
* doc/guix.texi (Invoking guix system): Document '--system' and
  '--image-size'.

doc/guix.texi

@@ -3445,7 +3445,21 @@ using the following command:
 @end table
 
 @var{options} can contain any of the common build options provided by
-@command{guix build} (@pxref{Invoking guix build}).
+@command{guix build} (@pxref{Invoking guix build}).  In addition,
+@var{options} can contain one of the following:
+
+@table @option
+@item --system=@var{system}
+@itemx -s @var{system}
+Attempt to build for @var{system} instead of the host's system type.
+This works as per @command{guix build} (@pxref{Invoking guix build}).
+
+@item --image-size=@var{size}
+For the @code{vm-image} and @code{disk-image} actions, create an image
+of the given @var{size}.  @var{size} may be a number of bytes, or it may
+include a unit as a suffix, such as @code{MiB} for mebibytes and
+@code{GB} for gigabytes.
+@end table
 
 Note that all the actions above, except @code{build} and @code{init},
 rely on KVM support in the Linux-Libre kernel.  Specifically, the

guix/scripts/system.scm

@@ -131,11 +131,12 @@ When GRUB? is true, install GRUB on DEVICE, using GRUB.CFG."
   ;; The system profile.
   (string-append %state-directory "/profiles/system"))
 
-(define* (switch-to-system store os system
-                           #:optional (profile %system-profile))
-  "Make a new generation of PROFILE pointing to SYSTEM, which is the directory
-corresponding to OS, switch to it atomically, and then run OS's activation
-script."
+(define* (switch-to-system store os system-directory
+                           #:optional (profile %system-profile)
+                           #:key system)
+  "Make a new generation of PROFILE pointing to SYSTEM-DIRECTORY, which is the
+directory corresponding to OS on SYSTEM, switch to it atomically, and then run
+OS's activation script."
   (let* ((number     (+ 1 (generation-number profile)))
          (generation (generation-file-name profile number)))
     (symlink system generation)
@@ -144,7 +145,8 @@ script."
     (run-with-store store
       (mlet %store-monad ((script (operating-system-activation-script os)))
         (format #t (_ "activating system...~%"))
-        (return (primitive-load (derivation->output-path script)))))
+        (return (primitive-load (derivation->output-path script))))
+      #:system system)
 
     ;; TODO: Run 'deco reload ...'.
     ))
@@ -241,6 +243,10 @@ Build the operating system declared in FILE according to ACTION.\n"))
          (option '(#\n "dry-run") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'dry-run? #t result)))
+         (option '(#\s "system") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'system arg
+                               (alist-delete 'system result eq?))))
          %standard-build-options))
 
 (define %default-options
@@ -305,6 +311,7 @@ Build the operating system declared in FILE according to ACTION.\n"))
            (args     (option-arguments opts))
            (file     (first args))
            (action   (assoc-ref opts 'action))
+           (system   (assoc-ref opts 'system))
            (os       (if file
                          (read-operating-system file)
                          (leave (_ "no configuration file specified~%"))))
@@ -323,12 +330,13 @@ Build the operating system declared in FILE according to ACTION.\n"))
                                              #:disk-image-size size)))))
            (store    (open-connection))
            (dry?     (assoc-ref opts 'dry-run?))
-           (drv      (run-with-store store mdrv))
+           (drv      (run-with-store store mdrv #:system system))
            (grub?    (assoc-ref opts 'install-grub?))
            (old      (previous-grub-entries))
            (grub.cfg (run-with-store store
-                       (operating-system-grub.cfg os old)))
-           (grub     (package-derivation store grub))
+                       (operating-system-grub.cfg os old)
+                       #:system system))
+           (grub     (package-derivation store grub system))
            (drv-lst  (if grub?
                          (list drv grub grub.cfg)
                          (list drv))))
@@ -357,7 +365,8 @@ Build the operating system declared in FILE according to ACTION.\n"))
                             (operating-system-bootloader os)))))
           (case action
             ((reconfigure)
-             (switch-to-system store os (derivation->output-path drv))
+             (switch-to-system store os (derivation->output-path drv)
+                               #:system system)
              (when grub?
                (unless (install-grub grub.cfg device target)
                  (leave (_ "failed to install GRUB on device '~a'~%") device))))