Bump imgtool version to 1.6.0-rc1.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 10a217b..5e3ab91 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 = "1.6.0a3"
+imgtool_version = "1.6.0rc1"