install: Make /var/db and /mnt.

* guix/build/install.scm (directives): Add /var/db and /mnt.

guix/build/install.scm

@@ -81,8 +81,10 @@ (define (directives store)
     (directory "/var/log")                          ; for dmd
     (directory "/var/guix/gcroots")
     (directory "/var/empty")                        ; for no-login accounts
+    (directory "/var/db")                           ; for dhclient, etc.
     (directory "/var/run")
     (directory "/run")
+    (directory "/mnt")
     (directory "/var/guix/profiles/per-user/root" 0 0)
 
     ;; Link to the initial system generation.