Add support for scanning downstream kernel forks

[ffainelli]

We maintain a number of downstream kernels that typically contain vendor changes on top of a stable kernel branch (e.g: 4.9.135 etc.). Those kernel trees are managed with git, so we can scan those trees the same way the upstream and linux-stable trees are scanned.

Could you publish the scripts that do the scanning of Linux kernel trees such that it is possible to add "vendor" streams to the stream list?

Thank you very much!

[nluedtke]

Push this out a milestone while we work to decouple and lint the backend code.