submodule's platform configurations should be in the submodules

Currently qtbase.txt and qtdeclarative.txt for example are under qt5.git in coin/platform_configs/. This causes a problem when there's a typo in these files. They are after all manually typed. The commit will still pass just fine in qt5.git, as it doesn't read them, but consequently the submodule builds will fail due to the typo.

Suggesting we move the files to the submodules respectively. Before that we code support for that in Coin, so that it reads the platform_configs from the submodules as well, when building the submodules. That way qtbase builds would take care of verifying that the qtbase.txt is properly made.