Split Features field into Coin and Qt specific flags

The files default.txt and qt5.txt etc under qt5.git's platform_configurations have a field called "Features". It has combined both features for Coin like "packaging" and some features that are given as arguments for the configure command. Whenever we want to support a new configurations argument, we had to modify Coin first that did a conversion from the feature string to the actual argument for configure. This slows down development.

By splitting the Feature up between Coin's features and configure arguments, and by having configure arguments passed on 1:1 to the command line, we can give any argument to configure without having to modify Coin first.

As configure arguments are given 1:1 here, we can now add all configure arguments to these default.txt kind of files and remove the code from Coin. Meaning, we can clean up targetenvironments.py largely. For instance we don't have to guess in Coin if we should add "-xplatform android-clang" or "-xplatform android-g++" based on the compilers given in default.txt, because this line is also now mentioned in there.