Zero free space on rootfs so it's more compressible for auto-update

build_image

Index: build_image
diff --git a/build_image b/build_image
index 722d704f96f5ad70ccc30b8af16b78afa1dad360..4280bddb9c6fd62439feb71a1de205c2499c5a6c 100755
--- a/build_image
+++ b/build_image
@@ -503,6 +503,14 @@ update_base_packages() {
     -s "${STATEFUL_FS_DIR}" -e "${ESP_FS_DIR}"
 }
 
+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
+  sudo rm -f "${fs_mount_point}/filler"
+  sudo sync
+}
+
 create_base_image() {
   local image_name=$1
 
@@ -683,6 +691,10 @@ create_base_image() {
   # Create an empty esp image to be updated in by update_bootloaders.sh.
   ${SCRIPTS_DIR}/create_esp.sh --to="${ESP_FS_IMG}"
 
+  # Zero rootfs free space to make it more compressible so auto-update
+  # payloads become smaller
+  zero_free_space "${ROOT_FS_DIR}"
+
   cleanup
 
   trap delete_prompt EXIT