kbuild: make single target builds even faster
BugLink: https://bugs.launchpad.net/bugs/1866333
commit
b1fbfcb4a20949df08dd995927cdc5ad220c128d upstream.
Commit
2dffd23f81a3 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>