vyos-kernel: enable VXLAN kernel module for 586-vyos kernel version

Both the 586-vyos-virt and amd64-vyos kernel versions have the VXLAN
kernel module enabled but it's missing in the 586-vyos version.  Also
linked to the recent enhancement proposal in Bug #294

Bug #301 http://bugzilla.vyos.net/show_bug.cgi?id=301