Add support for overriding install config steps

Add QGC_DISABLE_INSTALLER_SETUP to CONFIG will override config in QGCInstaller.pri.
However, QGCInstaller.pri can be imported in the custom one. In this way
a custom build can generate it's own installer config or add pre and post steps.

qgroundcontrol.pro

@@ -1288,4 +1288,8 @@ contains (CONFIG, QGC_DISABLE_BUILD_SETUP) {
 # Installer targets
 #
 
-include(QGCInstaller.pri)
+contains (CONFIG, QGC_DISABLE_INSTALLER_SETUP) {
+    message("Disable standard installer setup")
+} else {
+    include(QGCInstaller.pri)
+}