Preps for 2.1.0-rc1 release

Update release notes and version numbers for 2.1.0-rc1 release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 249e23c..0b415ad 100644
--- a/scripts/imgtool/__init__.py
+++ b/scripts/imgtool/__init__.py
@@ -14,4 +14,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "2.0.0"
+imgtool_version = "2.1.0rc1"