[Buildroot] Repair boot from SD card on i.MX6 Sabre boards

[Vincent Stehlé]

Hi,

Here is a two-patches series, to repair the boot from SD card on
Freescale Sabre boards.

The first patch adapts the SD card creation script to the new versions
of sfdisk; a prerequisite to actually preparing an SD card and booting
from it.

The second patch switches the defconfigs to building a zImage instead of
an uImage, to reflect what u-boot in Freescale releases awaits nowadays.

This was build-tested for all the five touched defconfigs but I did only
verify the boot on i.MX6 quad Sabre AI.

Feedbacks and testers welcome!

Best regards,

V.