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/docs/release-notes.md b/docs/release-notes.md
index 1f07638..c78d439 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -35,8 +35,8 @@
 ### Zephyr-RTOS Compatibility
 
 This release of MCUboot works the Zephyr "master" at the time of the
-release.  It was tested as of has aff616de20.  When Zephyr v2.3.0 is
-release, there will be a possible 1.6.1 or similar release of Zephyr
+release.  It was tested as of has 1a89ca1238.  When Zephyr v2.3.0 is
+released, there will be a possible 1.6.1 or similar release of Zephyr
 if needed to address any issues.  There also may be branch releases of
 MCUboot specifically for the current version of Zephyr, e.g.
 v1.6.0-zephyr-2.2.1.