imgtool: ECDSA pad signature

Until we fix the padding problem, pad the ecdsa signatures with zeros to
always be 72 bytes long.  Add a 'raw_sign' method so that the unit test
can use the real signature.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/keys/ecdsa_test.py b/scripts/imgtool/keys/ecdsa_test.py
index 326d73f..8cb3220 100644
--- a/scripts/imgtool/keys/ecdsa_test.py
+++ b/scripts/imgtool/keys/ecdsa_test.py
@@ -79,7 +79,7 @@
     def test_sig(self):
         k = ECDSA256P1.generate()
         buf = b'This is the message'
-        sig = k.sign(buf)
+        sig = k.raw_sign(buf)
 
         # The code doesn't have any verification, so verify this
         # manually.