gen: create a root logger in the root namespace

[doudou]

There is currently no way to reliably define loggers in the hierarchy, and that sucks. Big time.

I can see would be a few different ways:

1. in config/init.rb
2. in a lib/<app_name>.rb file
3. "internally" by the Roby app
4. would be the most consistent, but it is currently not loaded in dependencies (i.e. if rock_bla depends on rock, bundles/rock/config/init.rb is not loaded). 3. would be the easiest.