kbuild: Do not package /boot and /lib in make tar-pkg

There were reports of users destroying their Fedora installs by a kernel
tarball that replaces the /lib -> /usr/lib symlink. Let's remove the
toplevel directories from the tarball to prevent this from happening.

Reported-by: Andi Kleen <andi@firstfloor.org>
Suggested-by: Ben Hutchings <ben@decadent.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>