imx6sabresd: boot to /init in mfgtools mode

Buildroot supplies a nice /init wrapper script to use when booting from a
ramdisk.

We add a patch to u-boot to tell the kernel to boot into /init (instead of
/linuxrc) on i.MX6, when booting in mfgtools mode. This way we can boot a
buildroot system entirely through USB.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
[Luca: rebase on top of iMX6DL patches and patch iMX6DL defconfig as well]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
[Luca: build- and run-tested on i.MX6DL SABRESD]
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin+buildroot@gmail.com>
Cc: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>