freescale/imx6-sabresd: lock down kernel headers to the kernel version

The current defconfigs for iMX6 SABRESD kernel 3.10.17 from the Freescale
git repository. This kernel is older than the toolchain kernel headers
used in Buildroot (currently 3.16.3).

Explicitly lock down the kernel headers to match the version of the kernel
that is built.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>