Allow including yaml files in coin/platform_configs/foo.yaml

It would be useful to allow inclusion of .yaml files inside the platform configs yaml files.

For example i'd like to create a cmake_configs.yaml file, which i can include in qtbase.yaml, qtdeclarative.yaml, qtsvg.yaml, qtgraphicaleffects.yaml, etc

Currently i need to resort to a bunch of copy-pasting, which introduces possibilities for errors.

I can't add the configs to qt5.yaml, because not all repos have cmake build system files.