Preparing for v1.5.0-rc1

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index c20fb4f..d4c08b8 100644
--- a/scripts/imgtool/__init__.py
+++ b/scripts/imgtool/__init__.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "1.5.0a2"
+imgtool_version = "1.5.0rc1"