Add allowed hash for ECDSA256P1Public key

Author: Salamist

Diff for: scripts/imgtool/image.py

@@ -183,6 +183,7 @@ def tlv_sha_to_sha(tlv):
     keys.ECDSA384P1         : ['384'],
     keys.ECDSA384P1Public   : ['384'],
     keys.ECDSA256P1         : ['256'],
+    keys.ECDSA256P1Public   : ['256'],
     keys.RSA                : ['256'],
     keys.RSAPublic          : ['256'],
     # This two are set to 256 for compatibility, the right would be 512