scripts/unit-test: Only build coverage output if coverage run
scripts/unit-test: Only build coverage output if coverage run

If our project has no test cases run then no coverage data will be
produced. This results in a lack of .gcda files which causes the
coverage-html target to fail being built. Just avoid running the
coverage-html generation if we don't have coverage data.

Change-Id: If4342798c6bfaa4fd2eed77fe510fa6a70546fa4
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts