Update Config.xcconfig

Add ability to create a permanent override file for signing the app as is done in Loop (dev branch) and FreeAPS (freeaps_dev branch).
Look up 2 levels for an OverrideConfig.xcconfig file and use that if it exists. 

Note, because the name differs from that used for Loop, there is no conflict if both files are in the same folder.

This will simplify life for anyone using a script to build and for any developer who might have several different test branches going.
For example: I have a copy in my ~/Downloads/ManualClones folder and do all clones one level down from that, e.g., ~/Downloads/ManualClones/FAX-descriptor-date/freeapsx

Config.xcconfig

@@ -5,3 +5,4 @@ BUNDLE_IDENTIFIER = ru.artpancreas.$(DEVELOPMENT_TEAM).FreeAPS
 APP_GROUP_ID = group.com.$(DEVELOPMENT_TEAM).loopkit.LoopGroup
 
 #include? "ConfigOverride.xcconfig"
+#include? "../../ConfigOverride.xcconfig"