Add new verify command

imgtool verify -k <some-key.(pub|sec)> <img-file>

Allow imgtool to validate that an image has a valid sha256sum and that
it was signed by the supplied key.

NOTE: this does not yet support verifying encrypted images

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/keys/ecdsa.py b/scripts/imgtool/keys/ecdsa.py
index f541d16..f93783d 100644
--- a/scripts/imgtool/keys/ecdsa.py
+++ b/scripts/imgtool/keys/ecdsa.py
@@ -57,6 +57,14 @@
         # signature.
         return 72
 
+    def verify(self, signature, payload):
+        k = self.key
+        if isinstance(self.key, ec.EllipticCurvePrivateKey):
+            k = self.key.public_key()
+        return k.verify(signature=signature, data=payload,
+                        signature_algorithm=ec.ECDSA(SHA256()))
+
+
 class ECDSA256P1(ECDSA256P1Public):
     """
     Wrapper around an ECDSA private key.