images_types: isolate the write of UBI configuration

Isolate the writing of the UBI configuration in a separate function.
This allows to use a custom configuration easily by overriding the
function.

Replace the echo functions by a cat to have a more readable configuration.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>