gooftool: More fix for HasFpmcu()

HasFpmcu() is currently broken, because for the boards without FPMCU,
'cros_config fingerprint board' returns an empty string but still
succeeds.

BUG=b:145560202
TEST=make test
TEST=manually run finalization on a board with FPS and another board
without FPS.

Change-Id: I015cd72ad8fd8b2680431828d221f5bbfdc54724
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/1948163
Reviewed-by: Yong Hong <yhong@chromium.org>
Reviewed-by: Alex Chou <alex.chou@quanta.corp-partner.google.com>
Commit-Queue: Philip Chen <philipchen@chromium.org>
Tested-by: Philip Chen <philipchen@chromium.org>
diff --git a/py/gooftool/commands.py b/py/gooftool/commands.py
index 6af25a2..4934b92 100755
--- a/py/gooftool/commands.py
+++ b/py/gooftool/commands.py
@@ -25,7 +25,6 @@
 import xmlrpclib
 
 from six import iteritems
-from six.moves import xrange
 
 import factory_common  # pylint: disable=unused-import
 from cros.factory.gooftool.common import ExecFactoryPar
@@ -82,7 +81,8 @@
     FPMCU_PATH = '/dev/cros_fp'
     has_fpmcu_path = os.path.exists(FPMCU_PATH)
     has_cros_config_fpmcu = False
-    if Shell(['cros_config', '/fingerprint', 'board']).success:
+    cros_config_output = Shell(['cros_config', '/fingerprint', 'board'])
+    if cros_config_output.success and cros_config_output.stdout:
       has_cros_config_fpmcu = True
 
     if has_fpmcu_path is False and has_cros_config_fpmcu is True: