RELEASE_LVRT_COMMS-4.0: nati_x86_64_defconfig: build squashfs & overlayfs into the kernel

Switch squashfs from a module to a built in component of the kernel.
Also add overlayfs support to the kernel.  The new file layout requires
squashfs and overlayfs so they should be built in.

Signed-off-by: Bill Pittman <bill.pittman@ni.com>
Acked-by: Gratian Crisan <gratian.crisan@ni.com>
Natinst-ReviewBoard-ID: 294993