Better support for Git sources

[matteodelabre]

Some recipes may want to fetch their sources directly from Git, potentially including some submodules (see the fuse recipe in Toltec, or the fbink recipe, for example). Right now, the available options are: using some feature of the Git hosting that allows downloading source archives (but this doesn’t include submodules on GitHub, for example), or manually invoking git in prepare(). We should make our source= field support those use-cases to reduce code duplication in recipes.

See Arch’s implementation or OpenWrt’s implementation for reference.