uboot-sign: fix loop in do_uboot_assemble_fitimage

When using multiple u-boot configurations in UBOOT_CONFIG, the helper
function uboot_assemble_fitimage_helper() was not called with all
combinations of type & binary, due to a copy-n-paste indexing error.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>