image_types: fix multiubi var init

image_types: fix multiubi var init

Make sure to expand all MKUBIFS_ARGS_<label> and UBINIZE_ARGS_<label> vars
in 'do_image_multiubi' task to use them to init the local 'mkubifs_args'
and 'ubinize_args' vars.

See [YOCTO #15065]

Signed-off-by: Romuald JEANNE <romuald.jeanne@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 09d05215cf61981c7bc828cc0ff64c2fd5edc43c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>