use "--format gnutar" for bsdtar when creating preinstallimage

as this excludes the xattrs and unpacking xattrs is broken on
filesystems not supporting it as tmpfs and also seems broken on
bigendian platforms

build

@@ -2613,7 +2613,7 @@ for SPECFILE in "${SPECFILES[@]}" ; do
 	cd $BUILD_ROOT || cleanup_and_exit 1
 	TAR="tar"
 	if test -x /usr/bin/bsdtar; then
-	    TAR="/usr/bin/bsdtar --chroot"
+	    TAR="/usr/bin/bsdtar --format gnutar --chroot"
 	fi
 	TOPDIRS=
 	for DIR  in .* * ; do