west: fix import error on sign.py

Fix broken imports caused by changes
from its dependencies.

Signed-off-by: Jun Li <jun.r.li@intel.com>
diff --git a/scripts/west_commands/sign.py b/scripts/west_commands/sign.py
index 151a71a..9bac260 100644
--- a/scripts/west_commands/sign.py
+++ b/scripts/west_commands/sign.py
@@ -15,8 +15,11 @@
 
 from runners.core import BuildConfiguration
 
-from zephyr_ext_common import find_build_dir, Forceable, \
-    BUILD_DIR_DESCRIPTION, cached_runner_config
+from build_helpers import find_build_dir, \
+    BUILD_DIR_DESCRIPTION
+
+from zephyr_ext_common import Forceable, \
+    cached_runner_config
 
 SIGN_DESCRIPTION = '''\
 This command automates some of the drudgery of creating signed Zephyr