fix #675 ECDSA verification fail when padding is added

Signed-off-by: Anthony Liu <antliu@gmail.com>
diff --git a/scripts/imgtool/keys/ecdsa.py b/scripts/imgtool/keys/ecdsa.py
index c1c1cac..1fe805e 100644
--- a/scripts/imgtool/keys/ecdsa.py
+++ b/scripts/imgtool/keys/ecdsa.py
@@ -61,6 +61,8 @@
         return 72
 
     def verify(self, signature, payload):
+        # strip possible paddings added during sign
+        signature = signature[:signature[1] + 2]
         k = self.key
         if isinstance(self.key, ec.EllipticCurvePrivateKey):
             k = self.key.public_key()