freescale/imx6-sabresd: document how to create a bootable SD

Document how to create a bootable SD card for the two supported SABRESD cards:
i.MX6Q and i.MX6DL.

The SD card creation relies on an ad-hoc script.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Vincent Stehlé <vincent.stehle@freescale.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>