ci: uploading coverage to codecov.io

[glimchb]

see example : https://app.codecov.io/gh/glimchb/spdk
and badge :

[karlatec]

Unrelated change?
Summary requires a lot of inputs from previous stages, otherwise it's going to fail.

[karlatec]

I think you uploaded the artifact to codecov using a fork of SPDK Github repo as the code base, correct?
In that case this works all fine:
I can go to https://app.codecov.io/gh/glimchb/spdk/commit/ed1cc0a7aeff5ad5ef94cc200c6133158b0e9523 and then click 7 months ago glimchb authored commit ed1cc0a, which takes me to related commit in your SPDK fork. Using SPDK repo from Github also makes it possible for CodeCov to show coverage diff.

But there's going to be an issue with uploading & comparing coverage statistics for changes which are still in review (i.e. on Gerrit). In this case Codecov get's confused and can't fetch commit message, author, resolve history, etc.
See for example:
https://app.codecov.io/gh/karlatec/spdk/commits
https://app.codecov.io/gh/karlatec/spdk/commit/a5d76ac2bf379a48939fc174591d42c3f408f618

I'm not "against", just pointing this out.
Using Codecov for generating coverage statistics on already merged code (e.g. as part of some "nightly" tests) is going to be easy. Integrating it with Gerrit to deliver coverage-diff information for each patch is going to take a bit more work.