xserver-common: start Xsession when starting Xserver

xserver-common: start Xsession when starting Xserver

* since this commit
  commit 305038f932fbcb39f3bb6698f8fad18172c30962
  Author: Jonathan Liu <net147@gmail.com>
  Date:   Wed Jun 12 08:30:15 2013 +0000
    xserver-nodm-init: do not execute Xsession twice

  xserver-nodm-init wasn't working correctly when it was used together
  with xserver-common which doesn't start Xsession from Xserver (unlike
  x11-common in oe-core).

(From meta-openembedded rev: 619af4afb943fed12ef150f7ecf540776fee435e)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>