kernel-fitimage: Don't use unit addresses on FIT

Das U-Boot 2021.4-rc1 has the following commit:

    commit 3f04db891a353f4b127ed57279279f851c6b4917
    Author: Simon Glass <sjg@chromium.org>
    Date:   Mon Feb 15 17:08:12 2021 -0700

        image: Check for unit addresses in FITs

        Using unit addresses in a FIT is a security risk. Add a check for
        this and disallow it.

        CVE-2021-27138

Adjust the kernel-fitimage.bbclass accordingly to not use unit
addresses. This changte is required before we can bump U-Boot to 2021.4.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
[Backport for Dunfell]
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>