image_signing: do not fail when chronos does not exist

If the device doesn't create a chronos user, don't throw errors.
For some embedded systems, they don't need a chronos user.

BRANCH=None
BUG=chromium:714598
TEST=signing images still works

Change-Id: I4604beae1e647e024a04583471b8a7d0d4f188fa
Reviewed-on: https://chromium-review.googlesource.com/500027
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>

scripts/image_signing/common_minimal.sh

@@ -365,7 +365,10 @@ ensure_files_exist() {
 # Args: rootfs
 no_chronos_password() {
   local rootfs=$1
-  sudo grep -q '^chronos:\*:' "$rootfs/etc/shadow"
+  # Make sure the chronos user actually exists.
+  if grep -qs '^chronos:' "${rootfs}/etc/passwd"; then
+    sudo grep -q '^chronos:\*:' "${rootfs}/etc/shadow"
+  fi
 }
 
 trap "cleanup_temps_and_mounts" EXIT