add code coverage report make option

[nshaheed]

The COVERAGE_REPORT option will set the proper build/linking options to track code coverage and the make coverage command will then generate a coverage report from the unit tests.

example usage:

> make linux-all -j COVERAGE_REPORT=true
> make coverage

And it will spit out some nice htmls pages of our test coverage in the report/ directory.

Note: this add dependencies on gcov and lcov to generate the coverage reports, but will not affect building if you don't set teh COVERAGE_REPORT flag.

[gewang]

whole new world