Make submodules path absolute

Current submodule paths are relative. Hence, if you have your own fork of skyward-boardcode, this will break all submodules. This patchsets tries to solve this issue adding absolute paths to .gitmodules. The downside is that whenever the path of the repo changes this has to be changes manually. Arguably, the repo should not change paths unless there is a very good reason for that, so I think it is ok, your call.