build_image: silence the "No space left on device" message

build_image

Index: build_image
diff --git a/build_image b/build_image
index c54013bc4e6a58374fea4945e8a67ee4078b0c95..23c462ce40cb6e931f16eba051a9b27d4677bd09 100755
--- a/build_image
+++ b/build_image
@@ -523,7 +523,10 @@ update_base_packages() {
 zero_free_space() {
   local fs_mount_point=$1
   info "Zeroing freespace in ${fs_mount_point}"
-  sudo dd if=/dev/zero of="${fs_mount_point}/filler" oflag=sync bs=4096 || true
+  # dd is a silly thing and will produce a "No space left on device" message
+  # that cannot be turned off and is confusing to unsuspecting victims.
+  ( sudo dd if=/dev/zero of="${fs_mount_point}/filler" oflag=sync bs=4096 \
+    || true ) 2>&1 |grep -v "No space left on device"
   sudo rm -f "${fs_mount_point}/filler"
   sudo sync
 }