FFI os depency won't install

[Rauldg]

A package I am using depends on the libffi-dev. The dependency defintiion is here for instance.

The problem is that in the rock.osdeps that is generated by this package the entry of ffi is not complete:

ffi:
  default: ruby-ffi

In my manifest the rock-osdeps-package-set is the last one, so the complete definitions are overwritten and the library libffi-dev does not get installed by Autoproj.

If I set the rock-osdeps-package-set above any other one defining the osdep completely, then the dependency gets installed.

@2maz