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>