Issue 1.7.2-rc1 release

Even though the changes are small, issue an rc1 so that we have some
time to do testing on this release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 9855c37..a15e783 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 = "1.7.2"
+imgtool_version = "1.7.2rc1"