meson support: remove code warnings 2

This commit contains code changes necessary to support the increased
warning level from Meson builds. Most changes are for unused variables.
to keep the review size manageable, this commit contains only control
changes (plus one in sensor-monitor).

Change-Id: Ie20f1d9028add4b605e4cc9fb230940710365706
Signed-off-by: Mike Capps <mikepcapps@gmail.com>
22 files changed