Add versioning to imgtool

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/setup.py b/scripts/setup.py
index 0eeeba5..a58e6b0 100644
--- a/scripts/setup.py
+++ b/scripts/setup.py
@@ -1,8 +1,9 @@
 import setuptools
+from imgtool import imgtool_version
 
 setuptools.setup(
     name="imgtool",
-    version="1.3.1",
+    version=imgtool_version,
     author="The MCUboot commiters",
     description=("MCUboot's image signing and key management"),
     license="Apache Software License",