Use QMAKE_TARGET_BUNDLE_PREFIX to build bundle identifier

Currently the bundle identifier is build using com.yourcompany +
QMAKE_BUNDLE. This patch adds the handling of
QMAKE_TARGET_BUNDLE_PREFIX to build the bundle prefix.

Task-number: QTBUG-19006

Change-Id: I014279da6dbef393b0df36f6d4995e40ab105316
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>