webos.conf: change preferred version of linux-yocto to 3.4

* 3.4 is default version used in Yocto 1.3
* move P_V_linux-yocto from webos.conf to
  webos-preferred-versions.inc
* no need to define the same P_V_linux-yocto for each MACHINE

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Change-Id: I3dc398e5f094e9d49bad9c0a7b62fa91f9dc8656
Reviewed-on: http://g2g.palm.com/1106
Reviewed-by: Susan Montooth <susan.montooth@palm.com>
Tested-by: Susan Montooth <susan.montooth@palm.com>