Update to 1.9.0-rc2

Bump the version number in the readme, and imgtool for the 1.9.0-rc2
release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index b7e5865..466f3b2 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 = "1.9.0rc1"
+imgtool_version = "1.9.0rc2"