configs/imx6qsabreauto: Use zImage by default

Currently the generated sdcard.img does not boot because U-Boot looks for
a zImage type of kernel instead of uImage.

Remove the generation of uImage, so that the sdcard.img can successfully
boot.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>