Targets "clean" and "install" run make recursively in a for loop.
This ignores -j and -k.  Target "all" depends on SUBDIR/all to recurse
into each SUBDIR.  Behaves nicely with -j and -k.  Put that to use for
"clean" and "install": depend on SUBDIR/clean or SUBDIR/install,
respectively, and delete the loop.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190528082308.22032-5-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>