Built-Using was missing some items, see #696418 & #700026, so a patch
was pushed to try and deal with it:

    commit b4411bfb56601cf6a7366a1bec065b56b91f1a42
    Author: Cyril Brulebois <kibi@debian.org>
    Date:   Mon Mar 3 00:00:46 2014 +0100

        Extend write-built-using to also generate the ${extra:Built-Using} substvar.

        This makes it possible to track packages mentioned in the EXTRA_PACKAGES
        variable (currently set to "bf-utf-source syslinux"). Thanks to Ansgar
        Burchardt for the reports (Closes: #696418, #700026).

        Note: Unknown packages are ignored, so architecture-specific packages
        shouldn't be a problem.

        Acked-by: Ian Campbell <ijc@debian.org>
        Signed-off-by: Cyril Brulebois <kibi@debian.org>

I lost track of Karsten's objection here:
https://lists.debian.org/debian-boot/2015/02/msg00012.html

so here's a bug report to try and get that fixed instead of just
reverting the said commit. (Funny how things that shouldn't be an issue
turn out to be one…)

It's indeed sad that I only tested the case of an entirely inexistent
package and didn't think about the funny not-available-on-all-archs
packages…

Maybe we could iterate over the packages mentioned there, calling
dpkg-checkbuilddeps with its -d flag to see if the relevant packages are
relevant? Another idea might be to use Dpkg::Deps to figure out whether
some package is relevant for the current architecture? Any other ideas?

I'd happily take patches here, as I'm busy with other bugs.

Mraw,
KiBi.

Cyril Brulebois <kibi@debian.org> (2015-03-09):

Karsten, I can't seem able to reproduce that in an armhf sid chroot on
harris.debian.org… Any clues? I'd like to upload src:debian-installer
really soon, and that's the only thing remaining for now.

Mraw,
KiBi.

I have run some further tests in the meantime. Those have shown
that the effect happens on my local armhf build box when I build
"by hand", but not when I use pbuilder.

The significant difference between both is that I have arch:all
packages built by the syslinux source package (isolinux and
syslinux-common) installed on my system, but not in the chroot
used by pbuilder.  If I uninstall those arch:all packages, the
effect does not occur anymore.  This means that building on a
buildd will probably be safe for now, but if one architecture
should in the future add a built-using for an arch-specific
package, whose source also builds some arch:all package that is a
build-dependency of the installer on another platform, the
problem would probably crop up on the buildds as well.  With the
current setup, I think the chance that this will actually happen
in the forseeable future is rather low, but it would of course be
better to find a "proper" solution for the issue.

Regards,
Karsten

Karsten Merker <merker@debian.org> (2015-03-30):

OK, thanks for the extra checks.

Keeping this bug report open at important severity and bumping it to
serious whenever it becomes a practical issue looks like a reasonable
thing to do, so let's do that.

Mraw,
KiBi.