kbuild: generate modules.order only in directories visited by obj-y/m

The modules.order files in directories visited by the chain of obj-y
or obj-m are merged to the upper-level ones, and become parts of the
top-level modules.order. On the other hand, there is no need to
generate modules.order in directories visited by subdir-y or subdir-m
since they would become orphan anyway.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>