Make the path to Yocto tool chain's environment setup file part of the Kits

When using Yocto cross compilation tool chains - under Linux (Ubuntu) - we have to run the tool chains environment-setup-* file - using the source command - in a terminal, and then start Qt Creator from the command line in the same terminal, to make the exported variables available in Qt Creators build environment. These exported variables must be available when make is executed. It would be nice if the path to this environment-setup-* file could be set as a part of the Kit in Qt Creator, so that changing the current Kit also changed the Build Environment variables accordingly, since we have several different Yocto targets. The path would have to include the name of the file, since that changes from tool chain to tool chain.

The attached file is an example of an environment-setup-* file generated by the installation of the cross compilation tool chain.