CI: preprocess coverage with lcov for codecov.io

We preprocess gcov output with lcov before feeding it to
codecov. For the following reasons:
* codecov seems to support LCOV pseudo-pragmas only if
  they match the pattern "// LCOV_EXCL"
* It is easier to understand what will happen in codecov
  if we feed a preprocessed lcov file than a pile of raw
  gcov files
* We can reproduce that step locally and therefore do
  quicker trials
* Branch coverage is not correct when feeding gcov raw
  input to codecov

+ We avoid feeding gcovr's output to codecov:
Codecov upload bash script also sees shippable/codecoverage/coverage.xml
but when this is uploaded together with lcov's the branch coverage
is not correct, and some not compiled in files are reported as not
covered.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>