This is the first time i am editing a Makefile :).

This allowed to avoid the "dynamic" check of `SKIP_SUBMODULE_CHECK` later.

I just thought that it was a good separation of concerns: when declaring dependencies of bootstrap stages, it should not be necessary to know where the source comes from.  (Also, `check-submodules` IMO should not check if it is in a Git repository -- it should assume that it is in a Git repository.)

Setting `SKIP_SUBMODULE_CHECK` variable did not look right to me, because if not in Git repository, there is nothing to skip.  Maybe it is ok though.  What do you think?