imgtool.py: Fix some errors in ecdsa key generation

There are two easy to fix problems with the ecdsa key generation code.

Signed-off-by: David Brown <david.brown@linaro.org>

scripts/imgtool.py

@@ -9,7 +9,7 @@ import sys
 def gen_rsa2048(args):
     keys.RSA2048.generate().export_private(args.key)
 def gen_ecdsa_p256(args):
-    keys.ecdsa256p1.generate().export_private(args.key)
+    keys.ECDSA256P1.generate().export_private(args.key)
 def gen_ecdsa_p224(args):
     print("TODO: p-224 not yet implemented")
 

scripts/imgtool/keys.py

@@ -84,7 +84,7 @@ class ECDSA256P1():
 
     def export_private(self, path):
         with open(path, 'wb') as f:
-            f.write(key.to_pem())
+            f.write(self.key.to_pem())
 
     def emit_c(self):
         vk = self.key.get_verifying_key()