kbuild: make single target builds even faster

Commit 2dffd23f ("kbuild: make single target builds much faster")
made the situation much better.

To improve it even more, apply the similar idea to the top Makefile.
Trim unrelated directories from build-dirs.

The single build code must be moved above the 'descend' target.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jens Axboe <axboe@kernel.dk>