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.mk b/core/product.mk
index adc81c3..a9a24d2 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -68,7 +68,8 @@
     PRODUCT_SDK_ADDON_NAME \
     PRODUCT_SDK_ADDON_COPY_FILES \
     PRODUCT_SDK_ADDON_COPY_MODULES \
-    PRODUCT_SDK_ADDON_DOC_MODULE
+    PRODUCT_SDK_ADDON_DOC_MODULE \
+    PRODUCT_DEFAULT_WIFI_CHANNELS
 
 define dump-product
 $(info ==== $(1) ====)\