kbuild: Fix tar-pkg with relative $(objtree)

Commit 7e1c0477 (kbuild: Use relative path for $(objtree)) assumes that
the build process does not change its working directory. make tar-pkg
was a couterexample, fix this by changing directory only for the tar
command and not for the whole script, which at one point references the
now relative $(objtree).
Reported-and-tested-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/package/buildtar

@@ -125,12 +125,11 @@ esac
 # Create the tarball
 #
 (
-	cd "${tmpdir}"
 	opts=
 	if tar --owner=root --group=root --help >/dev/null 2>&1; then
 		opts="--owner=root --group=root"
 	fi
-	tar cf - boot/* lib/* $opts | ${compress} > "${tarball}${file_ext}"
+	tar cf - -C "$tmpdir" boot/ lib/ $opts | ${compress} > "${tarball}${file_ext}"
 )
 
 echo "Tarball successfully created in ${tarball}${file_ext}"