Since Toradex V2.7 SDK, U-Boot image has changed from u-boot.imx to
SPL + u-boot.img.
Also fix U-Boot build error with GCC6.
Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>