imgtool: Add password support to ECDSA

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool.py b/scripts/imgtool.py
index b2226db..bbd4110 100755
--- a/scripts/imgtool.py
+++ b/scripts/imgtool.py
@@ -21,7 +21,7 @@
 from imgtool import version
 import sys
 
-def gen_rsa2048(args):
+def get_password(args):
     if args.password:
         while True:
             passwd = getpass.getpass("Enter key passphrase: ")
@@ -34,11 +34,16 @@
         # encoding.
         return passwd.encode('utf-8')
     else:
-        passwd = None
+        return None
+
+def gen_rsa2048(args):
+    passwd = get_password(args)
     keys.RSA2048.generate().export_private(path=args.key, passwd=passwd)
 
 def gen_ecdsa_p256(args):
-    keys.ECDSA256P1.generate().export_private(args.key)
+    passwd = get_password(args)
+    keys.ECDSA256P1.generate().export_private(args.key, passwd=passwd)
+
 def gen_ecdsa_p224(args):
     print("TODO: p-224 not yet implemented")