Split hdiutil step separate from macdeployqt

hdiutil called from inside macdelpoyqt was failing. Managed to get it
not to fail by changing layout to SPUD instead of GPTSPUD.
Unfortunately know idea why this works!

QGCInstaller.pri

@@ -19,7 +19,8 @@
 
 installer {
     MacBuild {
-        QMAKE_POST_LINK += && $$dirname(QMAKE_QMAKE)/macdeployqt $${DESTDIR}/qgroundcontrol.app -dmg
+        QMAKE_POST_LINK += && $$dirname(QMAKE_QMAKE)/macdeployqt $${DESTDIR}/qgroundcontrol.app
+        QMAKE_POST_LINK += && hdiutil create -layout SPUD -srcfolder $${DESTDIR}/qgroundcontrol.app -volname qgroundcontrol $${DESTDIR}/qgroundcontrol.dmg
     }
     
     WindowsBuild {