system: Support activation service for the Hurd.

* gnu/build/activation.scm (boot-time-system): Use "command-line" for the
Hurd.
* gnu/system.scm (hurd-default-essential-services): Add %boot-service and
%activation-service.

gnu/build/activation.scm

@@ -320,7 +320,9 @@ improvement."
 
 (define (boot-time-system)
   "Return the '--system' argument passed on the kernel command line."
-  (find-long-option "--system" (linux-command-line)))
+  (find-long-option "--system" (if (string-contains %host-type "linux-gnu")
+                                   linux-command-line
+                                   (command-line))))
 
 (define* (activate-current-system
           #:optional (system (or (getenv "GUIX_NEW_SYSTEM")

gnu/system.scm

@@ -601,6 +601,8 @@ bookkeeping."
 
 (define (hurd-default-essential-services os)
   (list (service system-service-type '())
+        %boot-service
+        %activation-service
         (service profile-service-type '())))
 
 (define* (operating-system-services os)