imx6sabresd: rename to imx6sabre

Rename imx6sabresd board folder to imx6sabre, to prepare for Sabre Auto
addition. Update doc, link and defconfigs accordingly.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Eric Bénard <eric@eukrea.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>