Preps for 1.7.0-rc1

Update version fields for 1.7.0-rc1 release.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index ae25718..cd3b885 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 = "1.7.0a1"
+imgtool_version = "1.7.0rc1"