Current snapshot images doesn't provide images with usable overlay
filesystems:
mount_root: no usable overlay filesystem found, using tmpfs overlay
So this patch fixes this by updating support to current f2fs/ext4 based
overlays and providing squashfs with proper rootfs
TARGET_ROOTFS_PARTSIZE size.
While at it, I've also added support for optional gzipping of the
images, and renamed the output rootfs image filenames to match the more
common naming pattern $(IMG_PREFIX)-rootfs-$(FS-TYPE).img.
Cc: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
53f4a686aa