image_types: allow the creation of block devices on top of UBI volumes

Currently, the function multiubi_mkfs doesn't allow using anything else than
UBIFS over UBI.

Here, we propose to introduce two variables that allow to customize the ubinize
configuration and the image dependency, in order to be able to build UBI
images using the ubiblock abstraction layer and block filesystems.

For example, with this change it is possible to build a UBI volume using a
compressed squashfs, with the following configuration:
UBI_VOLTYPE ?= "static"
UBI_IMGTYPE ?= "squasfs-lz4"

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>