board/zynqmp/kria/kv260/kv260.sh: fix u-boot.itb without CONFIG_MULTI_DTB_FIT option

This patch fixes the kv260.sh to generate a working u-boot.itb
now that the CONFIG_MULTI_DTB_FIT u-boot option is no longer used.

This is a follow-up fix of
515319b8 ("board/zynqmp/kria/kv260/uboot.fragment:
remove unnecessary CONFIG_MULTI_DTB_FIT option") to fix the build of:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3310463281



Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>