build: Minor tidy up of outputexports

These changes act upon outstanding review comments when
commit 90f6ae15324b ("build: Support for integrating third party
build systems") was merged.

See https://gerrit.zephyrproject.org/r/#/c/4915/ for the comments.

Change-Id: I6b7287355f40658e47b94a8a7854776c173a7796
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
1 file changed