Feature/open logic fix

[rbrglez]

This pull request solves issue #24

[obruendl]

@olofk The implementation does seem to be inconsistent with the implementation. The documentation says paths are relative to .core files, the implementation interprets paths as relative to the repo. Please resolve the inconsistency - I am against accepting the pull request before the inconsistency is resolved - because the changes violate the documented behavior

See:

• [Doc] Source file paths in .core files are not according to documentation (not relative to the .core file) olofk/fusesoc#717
• https://fusesoc.readthedocs.io/en/stable/user/build_system/core_files.html#specifying-source-files

[obruendl]

The behavior is understood - fusesoc documentation regarding file paths for .core files with provider keys shall be updated but this is related to fusesoc, not fusesoc-cores.

This PR is redundant (the same changes are applied through #21 )

@rbrglez Hence I suggest you close this PR.