builddeb: Pass the kernel:debarch substvar to dpkg-genchanges

At the end of "make bindeb-pkg" I noticed the following warning:

dpkg-genchanges: warning: unknown substitution variable ${kernel:debarch}

It turns out that since dpkg version 1.19.0 dpkg-genchanges honors
substitution variables in the Description field, while earlier
versions silently left them alone, see https://bugs.debian.org/856547.

The result is an incomplete description of the linux-headers package
in the generated .changes file.  Fix it by passing the kernel:debarch
substitution variable to dpkg-genchanges.
Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/package/builddeb

@@ -408,9 +408,9 @@ EOF
 	dpkg-source -cdebian/control -ldebian/changelog --format="3.0 (custom)" --target-format="3.0 (quilt)" \
 		-b / ../${sourcename}_${version}.orig.tar.gz  ../${sourcename}_${packageversion}.debian.tar.gz
 	mv ${sourcename}_${packageversion}*dsc ..
-	dpkg-genchanges > ../${sourcename}_${packageversion}_${debarch}.changes
+	dpkg-genchanges -Vkernel:debarch="${debarch}" > ../${sourcename}_${packageversion}_${debarch}.changes
 else
-	dpkg-genchanges -b > ../${sourcename}_${packageversion}_${debarch}.changes
+	dpkg-genchanges -b -Vkernel:debarch="${debarch}" > ../${sourcename}_${packageversion}_${debarch}.changes
 fi
 
 exit 0