qt: updated qmake2 class to export qconfig.pri mkspec

This is necessary when Qt applicatins want to know variables such as
QT_MAJOR_VERSION, QT_VERSION, QT_MINOR_VERSION, gcc version etc.

OBS: This change made the build system crazy. When I changed it and run
bitbake my-qt-app-recipe, qmake was consuming 100% cpu usage for long time. I
had to stop it. So I recommend rebuild the entire Qt stack when changing this.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>