Add TARGET_USERNAME variable to define username on target distro. Layer can be now used by others also.

Set QT_DIR_NAME to an empty string so that Qt specific binaries are no longer installed in odd not-in-PATH places like /usr/bin/qt5/. It also removes the need for some ugly sed hacks. Cf: https://github.com/meta-qt5/meta-qt5/blob/master/classes/qmake5_paths.bbclass