qmake: Introduce CE_PLATFORMNAME to fix creation of vcxproj files

While previous SDKs used the form "CE_SDK (CE_ARCH)" in their
configuration/platform names, this is not true fo the Toradex SDK.
Inside Visual Studio the platform is only called "Toradex_CE800"
instead of "Toradex_CE800 (ARMV7)".

In order not to break other SDKs CE_PLATFORMNAME is introduced and
used in the wince80colibri-armv7-msvc2012 mkspec. If the variable
is set qmake uses it as the platform name in its vcproj generator.

Change-Id: Icb501bf6446a9f617745a0d2c7a80b240680b043
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>