imgtool: Fix getpriv error return with private key

This commit fixes a bug with the getpriv command using
ECDSA keys.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I66c1365a855e97199921ac136a18e26988bce508

scripts/imgtool/keys/ecdsa.py

@@ -30,7 +30,7 @@ def _unsupported(self, name):
         raise ECDSAUsageError("Operation {} requires private key".format(name))
 
     def _get_public(self):
-        return self.key.public_key()
+        return self.key
 
     def get_public_bytes(self):
         # The key is embedded into MBUboot in "SubjectPublicKeyInfo" format
@@ -137,6 +137,9 @@ def export_private(self, path, passwd=None):
         with open(path, 'wb') as f:
             f.write(pem)
 
+    def _get_public(self):
+        return self.key.public_key()
+
 
 class ECDSA256P1Public(ECDSAPublicKey):
     """
@@ -179,7 +182,7 @@ def verify(self, signature, payload):
                         signature_algorithm=ec.ECDSA(SHA256()))
 
 
-class ECDSA256P1(ECDSA256P1Public, ECDSAPrivateKey):
+class ECDSA256P1(ECDSAPrivateKey, ECDSA256P1Public):
     """
     Wrapper around an ECDSA (p256) private key.
     """
@@ -252,7 +255,7 @@ def verify(self, signature, payload):
                         signature_algorithm=ec.ECDSA(SHA384()))
 
 
-class ECDSA384P1(ECDSA384P1Public, ECDSAPrivateKey):
+class ECDSA384P1(ECDSAPrivateKey, ECDSA384P1Public):
     """
     Wrapper around an ECDSA (p384) private key.
     """