Add versioning to imgtool

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 107921f..8f6e848 100644
--- a/scripts/imgtool/__init__.py
+++ b/scripts/imgtool/__init__.py
@@ -11,3 +11,5 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+imgtool_version = "1.4.0a3"