how to checkout proper submodules

[Jan Stolarek]

> 1) We could now delete ./sync-all if this happened.
In that case I would vote for replacing sync-all with a script that aids in managing branches in 
multiple subrepos. I implemented such a script for myself in a very ad hoc way. Having something 
more robust would be great.

> 2) One thing this *does* complicate is that currently, some
> repositories are optional.  (...)
I believe this could be solved by changes in the build system, so that some components can be 
optional (yes, I also delete DPH to speed up building).

Janek