Fix objdir

[Jarod42]

Currently, you use cfg.objdir before it is baked. (so you get "obj" instead of "obj/Release" and when nil you don't use the same algo than premake-core neither).

Currently, as default (i.e no qtgenerateddir provided), generated files are in same directory for all configs
(and according to you :) :

outputing generated files into the same folder/same name (which IMO is a bad practice, but that's probably just a matter of taste :))
)

So with that PR, we call qt.customAddFileConfig after oven.bakeObjDirs, so cfg.objdir is correct in qt.getGeneratedDir.

Note:
That currently breaks some generators (xcode4, Code::Blocks, Codelite) for default:
C::B doesn't handle different rule by configuration.
Similarly for xcode4, but I don't know if the limitation come from xcode4 or if it is a premake-core issue.
premake-core has issue with Codelite which generate a nil deferencement (which happen also before for some config)

Specifying appropriate qtgenerateddir avoids that issue in that case.