gnu: guix-configuration: Improve offload build-machines.

It’s currently difficult to programmatically add a build-machine, because the
`service-extension' mechanism is the only facility which can do that.  It
relies on the `guix-service-type', ala:

    (service-extension guix-service-type
      (guix-extension (build-machines (list ...))))

...but `guix-service-type' is already instantiated as part of
`%base-services', and replacing it may lose other configuration, like
substitute servers and authorized keys.

Additionally, a default value of `#f' for the build-machines field requires
guarding uses of the field with:

    (or (guix-build-machines config) '())

Changing the default to be the empty list avoids that.  One can now add
build-machines with code such as:

    (modify-services %base-services
      (guix-service-type
       config =>
       (guix-configuration
        (inherit config)
        (authorized-keys
         (cons %build-machine-key
               (guix-configuration-authorized-keys config)))
        (build-machines (cons #~(build-machine ...)
                              (guix-configuration-build-machines config))))))

* gnu/services/base.scm (guix-configuration): Rename `guix-build-machines' to
`guix-configuration-build-machines' and export it.  Change the default from
`#f' to the empty list.
* gnu/services/base.scm (guix-activation): Update the build-machines test and
reverse the conditions.

Change-Id: I6780c6a5579fd9d4b4f22ee2b2bf7ba7a0454407
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/base.scm

@@ -212,6 +212,7 @@
             guix-configuration-guix
             guix-configuration-build-group
             guix-configuration-build-accounts
+            guix-configuration-build-machines
             guix-configuration-authorize-key?
             guix-configuration-authorized-keys
             guix-configuration-use-substitutes?
@@ -1848,8 +1849,8 @@ archive' public keys, with GUIX."
                     (default #f))
   (tmpdir           guix-tmpdir                   ;string | #f
                     (default #f))
-  (build-machines   guix-build-machines           ;list of gexps | #f
+  (build-machines   guix-configuration-build-machines ;list of gexps | '()
-                    (default #f))
+                    (default '()))
   (environment      guix-configuration-environment  ;list of strings
                     (default '())))
 
@@ -2044,10 +2045,10 @@ proxy of 'guix-daemon'...~%")
         #$(and channels (install-channels-file channels))
 
         ;; ... and /etc/guix/machines.scm.
-        #$(if (guix-build-machines config)
+        #$(if (null? (guix-configuration-build-machines config))
+              #~#f
               (guix-machines-files-installation
-               #~(list #$@(guix-build-machines config)))
+               #~(list #$@(guix-configuration-build-machines config)))))))
-              #~#f))))
 
 (define-record-type* <guix-extension>
   guix-extension make-guix-extension
@@ -2093,9 +2094,9 @@ proxy of 'guix-daemon'...~%")
               (substitute-urls (append (guix-extension-substitute-urls extension)
                                        (guix-configuration-substitute-urls config)))
               (build-machines
-               (and (or (guix-build-machines config)
+               (and (or (guix-configuration-build-machines config)
                         (pair? (guix-extension-build-machines extension)))
-                    (append (or (guix-build-machines config) '())
+                    (append (guix-configuration-build-machines config)
                             (guix-extension-build-machines extension))))
               (chroot-directories
                (append (guix-extension-chroot-directories extension)