kbuild: make modules.order rule consistent with built-in.a

kbuild: make modules.order rule consistent with built-in.a

built-in.a contains the built-in object paths from the current and sub
directories.

module.order collects the module paths from the current and sub
directories.

Make their build rules look more symmetrical.

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