Fix qbsTargetPlatformFromQtMkspec for mkspecs in subdirectories
Fix qbsTargetPlatformFromQtMkspec for mkspecs in subdirectories

The function qbsTargetPlatformFromQtMkspec failed for mkspecs like
"devices/linux-oe-generic-g++". Now we only inspect the part after the
last slash.

Task-number: QBS-1328
Task-number: QTCREATORBUG-20136
Change-Id: I41cf686bd8eff4235ce86de30afa8730d3696abc
Reviewed-by: Eike Ziller <c142da16c73a95a82d35191bcc55b22835617128@qt.io>
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs