imgtool: Print image digest during verify

In an effort to create a script to do firmware based on an URL, I could
not easily retrieve the image digest. `newtmgr` needs the hash when
marking a image for testing.

Signed-off-by: Casper Meijn <casper@meijn.net>
diff --git a/scripts/imgtool/main.py b/scripts/imgtool/main.py
index 58564e5..8fee864 100755
--- a/scripts/imgtool/main.py
+++ b/scripts/imgtool/main.py
@@ -140,10 +140,11 @@
 @click.command(help="Check that signed image can be verified by given key")
 def verify(key, imgfile):
     key = load_key(key) if key else None
-    ret, version = image.Image.verify(imgfile, key)
+    ret, version, digest = image.Image.verify(imgfile, key)
     if ret == image.VerifyResult.OK:
         print("Image was correctly validated")
         print("Image version: {}.{}.{}+{}".format(*version))
+        print("Image digest: {}".format(digest.hex()))
         return
     elif ret == image.VerifyResult.INVALID_MAGIC:
         print("Invalid image magic; is this an MCUboot image?")