confifs/freescale_imx28evk_defconfig: select headers 3.19

We use a manually specified custom version 3.19.2 of the linux kernel
for i.MX28, but linux-headers defaults to a 2.6.x family.

Select custom headers family 3.19.x explicitely, to repair
linux-headers "build".

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>