kernel-fitimage: fix dtbo support for fit images

8a2f4e143 added support for u-boot boot script but missed adding the
extra parameter to fitimage_emit_section_config on the dtbo branch

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 22bac8ae)
Signed-off-by: Steve Sakoman <steve@sakoman.com>