gen_merge_report.sh: Actually use INFO_PATH/JSON_PATH settings

These config vars existed, but weren't actually used in the script. Use
them, with related refactors (like factoring out common artifact path
to another config var, ARTIFACT_PATH).

These changes are in preparation of supporting ArmCI vs OpenCI code
coverage related artifact path differences.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I9d696d3acd4dfe9f7d43f1961fede209ccc61190
1 file changed