Update to version 1.9.0

Bump version information for 1.9.0 release

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 466f3b2..a7f5978 100644
--- a/scripts/imgtool/__init__.py
+++ b/scripts/imgtool/__init__.py
@@ -14,4 +14,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "1.9.0rc2"
+imgtool_version = "1.9.0"