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/tools/buildinfo.sh b/tools/buildinfo.sh
index 1fcac76..532ff57 100755
--- a/tools/buildinfo.sh
+++ b/tools/buildinfo.sh
@@ -23,6 +23,7 @@
 echo "ro.product.manufacturer=$PRODUCT_MANUFACTURER"
 echo "ro.product.locale.language=$PRODUCT_DEFAULT_LANGUAGE"
 echo "ro.product.locale.region=$PRODUCT_DEFAULT_REGION"
+echo "ro.wifi.channels=$PRODUCT_DEFAULT_WIFI_CHANNELS"
 echo "ro.board.platform=$TARGET_BOARD_PLATFORM"
 
 echo "# ro.build.product is obsolete; use ro.product.device"