[Rivet] Move bootstrap into main repository for versioning?

[David Grellscheid]

> The 1.9.0/rivet-bootstrap-lcg should be taken from the
> ef0f9730654d/rivet-1-bootstrap-lcg (i.e. the same revision as before,
> just the other file).
> 
> After that the default and 2.0.0 branches can get the following changes:
> - deletion of rivet-1-bootstrap-lcg
> - renaming of rivet-2-bootstrap to rivet-bootstrap
> - renaming of rivet-2-bootstrap-lcg to rivet-bootstrap-lcg

OK, done. Please test!

  David