-device: create qdevice.pri only when build tree is initialized

-device: create qdevice.pri only when build tree is initialized

The build tree is not initialized when -help is passed to configure.
The mkspecs/ directory is not created and thus the mv fails results
with the following error:
mv: cannot move `.device.vars' to `/tmp/qtbase/mkspecs/qdevice.pri': No such file or directory

The solution is to create qdevice.pri just before we run config.tests.

Change-Id: Ie83018e27a03bb840d213aae8c963b0074e62bb4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>