env: remove vars that are not in default env

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>