imx6: convert ventana images to squashfs+ubifs overlay

imx6: convert ventana images to squashfs+ubifs overlay

Replace the ventana ubi images from a single rootfs volume containing the
rootfs as well as kernel+dtbs in the boot/ subdir to a multi-volume ubi
containing kernel+dtbs+bootscript in 'root' ubifs volume, rootfs in 'rootfs'
squasfs volume, and rootfs_data an empty ubifs volume.

At the same time move ventana image creation over to using the new
TARGET_DEVICE syntax as a cleanup.

Additionally removed FIT image creation as it is recommended that this be
done manually to produce signed FIT images if needed.

Note that a bootscript is needed for Ventana to change its default mtdparts
to rename the 2nd partition from 'rootfs' to 'ubi'. This is because there
are patches in OpenWrt kernels that try to auto-attach and auto-mount and these
get in the way if the mtd partition is named 'rootfs'.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47508 3c298f89-4303-0410-b956-a3cf2f4a3e73