build_wrapper: Refactor the path to the configuration files

Use one variable instead of duplicating the path for all the
calls to mkqt5bld.py

Change-Id: Ibd7b89927f17b7e51a7e6affb6bb19e8dbb996fe
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Package docs in a separated qt5_docs.7z file

So they can be used for modules other than Qt without having
to include the whole qt5_essentials.7z file.

Change-Id: I094a8fb74d0e0cf7e4c635dae2560f20b98d2cab
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>