dpkg: Only set DEB_HOST_ARCH in target case

If we don't do this, the sstate checksums vary for dpkg-native depending on which MACHINE is set and this is clearly incorrect. It leads to dpkg-native rebuilding far too often.

index 52a9bd7..e8d8a9b 100644
@@ -33,7 +33,7 @@ EXTRA_OECONF = "\

  "

 EXTRA_OECONF += "TAR=tar"
-EXTRA_OECONF += "DEB_HOST_ARCH=${DPKG_ARCH}" +EXTRA_OECONF_append_class-target = " DEB_HOST_ARCH=${DPKG_ARCH}"
 do_configure () {
     echo >> ${S}/m4/compiler.m4