layerscape: Change to combined rootfs on sd images

layerscape: Change to combined rootfs on sd images

At this moment layerscape images are ext4 only. It causes problem with
save changes durring sysupgrade and make "firstboot" and failsafe mode
useless.

This patch changes sd-card images to squashfs + f2fs combined images.
To make place, for saving config, kernel space ar now ext4 partition
with fit kernel.

This method of image generation is similar to rest of OpenWrt sd-card
targets.

Reviewed-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
[reword README, reword DEVICE_COMPAT_MESSAGE, keep original indent]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>