imgtool: release 1.6.0alpha1

Change-Id: I7ae2bff9d16c39f7cc5fee3a08437f0a25e7cddc
Signed-off-by: David Vincze <david.vincze@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 04a1b5e..e6c9ed9 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 = "1.5.0"
+imgtool_version = "1.6.0a1"