kbuild: rename multi-used-* to multi-obj-*

I think multi-obj-* is clearer, and more consistent with real-obj-*.

Rename as follows:

  multi-used-y  ->  multi-obj-y
  multi-used-m  ->  multi-obj-m
  multi-used    ->  multi-obj-ym

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>