env: remove vars that are not in default env

current env_set_default_vars() doesn't delete
var that are not in the imported env. hashtable
removes vars that are not in the imported
env but present in the current env only if H_NOCLEAR
flag is not set.

This change is to avoid passing H_NOCLEAR flag if
specific vars are passed to env_set_default_vars()

Without this change:
Marvell>> env default boot_mode
Marvell>>

With the change:
Marvell>> env default boot_mode
WARNING: 'boot_mode' not in imported env, deleting it!

Signed-off-by: Ravi Minnikanti <rminnikanti@marvell.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/env.rst b/doc/usage/cmd/env.rst
index 9629f97..b65d85b 100644
--- a/doc/usage/cmd/env.rst
+++ b/doc/usage/cmd/env.rst
@@ -79,7 +79,8 @@
 environment to their default values.
 
     var
-        list of variable name.
+        list of variable names. If variable is not part of default
+        environment, it is deleted with a warning message on console.
     \-a
         all U-Boot environment.
     \-f
@@ -309,6 +310,7 @@
 Reset environment variable to default value, in memory::
 
     => env default bootcmd
+    => env default ipaddr serverip
     => env default -a
 
 Save current environment in persistent storage::