Introduce imx6-sabreauto_defconfig that uses mainline U-Boot and
mainline kernel.

As mainline U-Boot supports SPL, it is possible to boot the same
sdcard.img in the three mx6sabreauto variants: imx6q, imx6dl and imx6qp.

Also update the readme.txt file to explain how to build an image for
mx6sabreauto boards using mainline U-Boot and kernel.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
[Thomas: regenerate gitlab-ci.yml file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>