Preps for v1.6.0 release

Update README, release notes the mynewt repository file, and imgtool for
the v1.6.0 release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index c0c3ef2..09549c7 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 = "1.6.0rc2"
+imgtool_version = "1.6.0"