This change adds support for private github FuseSoC core repositories.

To test out on a linux installation:

1. Create a .netrc file at ~/.netrc with the format below. NOTE: The file permissions MUST be 600 for the .netrc file or it will not be used.
2. Attempt to run a FuseSoC core target for a core in a private repository.

Example ~/.netrc file format:

machine github.com
    login your_github_username
    password ghp_your_classic_gh_token

A new token can be created in github at Settings → Developer settings → Personal access tokens. I've only tested with the classic github token type and the only scope necessary is the repo checkbox and it's children.

This change does not introduce any new dependencies for FuseSoC, does not introduce or modify any flags, and should be transparent (used when .netrc with 600 permissions is available and the github.com machine is present in the .netrc file, ignored otherwise).