imgtool: rollback to 1.7.0 release

Since there won't be anymore alpha releases, rollback to latest official
(from master). The next released versions now can be either 1.7.X from
v1.7-branch or X.Y.Zrc1 for the release candidate of next stable
version.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/__init__.py b/scripts/imgtool/__init__.py
index 5d7d181..4f19399 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 = "1.8.0a1"
+imgtool_version = "1.7.0"