configs/imx6q-sabresd_defconfig: Switch to SPL boot

U-Boot 2017.07 has switched to SPL boot, which allows the same binary to
boot on imx6dl, imx6q and imx6qp sabresd boards.

Switch to the SPL mechanism.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>