meta-google: gbmc-update: stop self-upgrade if url is missing

This change will stop self update if the url is empty. Let it fall
through the upgrade hook and return error.

Change-Id: I92118d1697f1b8474c5152e6ea52f7ddb36b5615
Signed-off-by: Yuxiao Zhang <yuxiaozhang@google.com>
diff --git a/meta-google/recipes-phosphor/flash/gbmc-update/40-gbmc-upgrade.sh b/meta-google/recipes-phosphor/flash/gbmc-update/40-gbmc-upgrade.sh
index 3ac4020..d671e81 100644
--- a/meta-google/recipes-phosphor/flash/gbmc-update/40-gbmc-upgrade.sh
+++ b/meta-google/recipes-phosphor/flash/gbmc-update/40-gbmc-upgrade.sh
@@ -28,6 +28,11 @@
 fi
 
 gbmc_upgrade_dl_unpack() {
+  if [ -z "${bootfile_url-}" ]; then
+    echo "bootfile_url is empty" >&2
+    return 1
+  fi
+
   echo "Fetching $bootfile_url" >&2
 
   # We only support tarballs at the moment, our URLs will always denote
@@ -74,8 +79,6 @@
 }
 
 gbmc_upgrade_hook() {
-  [ -n "${bootfile_url-}" ] || return 0
-
   local tmpdir
   tmpdir="$(mktemp -d)" || return
   if ! gbmc_upgrade_dl_unpack; then