[x86] bump to 2.6.39.4

Since linux gets developed on x86 more than all other hardware,
hardly makes sense to have our default x86 version be trailing
by over a year.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28082 3c298f89-4303-0410-b956-a3cf2f4a3e73