buildroot

configs/mx6udoo: Bump kernel and U-Boot versions

Bump the kernel to version 4.13.5 and U-Boot to 2017.09.

While at it, remove the custom scripts for generating the SD card image
and use the standard scripts instead.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>