Add PRODUCT_DEFAULT_WIFI_CHANNELS build var.

Allows the build to specify the default number of allowed
WIFI channels.  Part of the fix for 1625953.
diff --git a/core/Makefile b/core/Makefile
index fa0550c..21c1e73 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -118,6 +118,7 @@
 			PRODUCT_BRAND="$(PRODUCT_BRAND)" \
 			PRODUCT_DEFAULT_LANGUAGE="$(call default-locale-language,$(PRODUCT_LOCALES))" \
 			PRODUCT_DEFAULT_REGION="$(call default-locale-region,$(PRODUCT_LOCALES))" \
+			PRODUCT_DEFAULT_WIFI_CHANNELS="$(PRODUCT_DEFAULT_WIFI_CHANNELS)" \
 			PRODUCT_MODEL="$(PRODUCT_MODEL)" \
 			PRODUCT_MANUFACTURER="$(PRODUCT_MANUFACTURER)" \
 			PRIVATE_BUILD_DESC="$(PRIVATE_BUILD_DESC)" \