How make a 'core' repository in Github? I need make dependencies

I have a repository with a code in C that is designed as a core code, it is so basic that it works for various embedded devices. I would like to use that core in several more specific device repositories and that these repositories have a dependency on this 'core repository'.

I have been seeing that there are tools like submodules, however it is not useful to me because these core files should be directly in the root of the repository.

I would like to know if there is a tool in git or Github that has been designed for this type of application.

Thank you very much.

Solution

Resolved with subdirectory Git feature. Thanks