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

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.