imgtool: Bump version number to 1.7.2

Update the version number of imgtool to reflect 1.7.2 release of
mcuboot.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

scripts/imgtool/__init__.py

@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "1.7.1"
+imgtool_version = "1.7.2"