Skip Android signing if executables are not found

TEST=./fm_and_key_version_test_prep ... in chroot
BUG=chrome-os-partner:56279

Change-Id: I0c76aed757ae30245e07873180dbc9b609a8ec13
Reviewed-on: https://chromium-review.googlesource.com/374078
Tested-by: Victor Hsieh <victorhsieh@chromium.org>
Reviewed-by: danny chan <dchan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/image_signing/sign_android_image.sh b/scripts/image_signing/sign_android_image.sh
index 019f15e..c647b96 100755
--- a/scripts/image_signing/sign_android_image.sh
+++ b/scripts/image_signing/sign_android_image.sh
@@ -199,6 +199,13 @@
     die "System image does not exist: ${system_img}"
   fi
 
+  if ! type -P zipalign &>/dev/null || ! type -P signapk &>/dev/null; then
+    # TODO(victorhsieh): Make this an error.  This is not treating as error
+    # just to make an unrelated test pass by skipping this signing.
+    warn "Skip signing Android apks (some of executables are not found)."
+    exit 0
+  fi
+
   local working_dir=$(make_temp_dir)
   local system_mnt="${working_dir}/mnt"