Default ro.carrier to unknown

This was previously performed by init.c

Change-Id: Id34c636e06dfe2766446f909e9d05557163d812d
diff --git a/CleanSpec.mk b/CleanSpec.mk
index 1d9b9bb..481829a 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -119,6 +119,9 @@
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/obj/APPS/*)
 $(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/build.prop)
 
+# Changing where ro.carrier value is instantiated for system/build.prop
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/build.prop)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************