branch_coverage.sh: ignore clear-command errors

Open CI uses a docker container to build and produce coverage
reports but calling the 'branch_coverage.sh' command, which in turn
calls the 'clear' command, the following error is observed

    .
    .
    12:54:34 # ../qa-tools/coverage-tool/coverage-reporting/branch_coverage.sh:133: main() - [4,0,0] clear
    12:54:34 TERM environment variable not set.
    12:54:34 LAVA JOB RESULT: 1

To avoid this problem, pipe the clear command with 'true', effectively
ignoring any error coming from clear command.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>