linux-boot: Mount /dev as a devtmpfs from the start.

Suggested by Petter <petter@mykolab.ch>
and Mark H Weaver <mhw@netris.org>.
Reported by Duncan Keall <duncan@duncankeall.com>.

Partly fixes <http://bugs.gnu.org/19190> by populating /dev/mapper
early enough.

* gnu/build/linux-boot.scm (mount-essential-file-systems): Mount /dev as
  a devtmpfs.
  (move-essential-file-systems): Add /dev.
  (mount-root-file-system): Mount /rw-root/dev as a devtmpfs instead of
  calling 'make-essential-device-nodes'.
  (boot-system): Remove call to 'make-essential-device-nodes'.
* gnu/system/file-systems.scm (%devtmpfs-file-system): Remove.
* doc/guix.texi (File Systems): Adjust accordingly.

gnu/system/file-systems.scm

@@ -47,7 +47,6 @@
             %binary-format-file-system
             %shared-memory-file-system
             %pseudo-terminal-file-system
-            %devtmpfs-file-system
             %immutable-store
             %control-groups
             %elogind-file-systems
@@ -186,17 +185,6 @@ UUID representation."
     (type "binfmt_misc")
     (check? #f)))
 
-(define %devtmpfs-file-system
-  ;; /dev as a 'devtmpfs' file system, needed for udev.
-  (file-system
-    (device "none")
-    (mount-point "/dev")
-    (type "devtmpfs")
-    (check? #f)
-
-    ;; Mount it from the initrd so /dev/pts & co. can then be mounted over it.
-    (needed-for-boot? #t)))
-
 (define %tty-gid
   ;; ID of the 'tty' group.  Allocate it statically to make it easy to refer
   ;; to it from here and from the 'tty' group definitions.
@@ -282,8 +270,7 @@ UUID representation."
 (define %base-file-systems
   ;; List of basic file systems to be mounted.  Note that /proc and /sys are
   ;; currently mounted by the initrd.
-  (append (list %devtmpfs-file-system
-                %pseudo-terminal-file-system
+  (append (list %pseudo-terminal-file-system
                 %shared-memory-file-system
                 %immutable-store)
           %elogind-file-systems