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/product_config.mk b/core/product_config.mk
index 7cfa5f4..3237801 100644
--- a/core/product_config.mk
+++ b/core/product_config.mk
@@ -214,6 +214,9 @@
   PRODUCT_MANUFACTURER := unknown
 endif
 
+PRODUCT_DEFAULT_WIFI_CHANNELS := \
+	$(strip $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_DEFAULT_WIFI_CHANNELS))
+
 # Which policy should this product use
 PRODUCT_POLICY := $(strip $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_POLICY))