cmd: nvedit: rename flags in do_env_default

cmd: nvedit: rename flags in do_env_default

The naming convention for flags in nvedit.c is:
* The hashtable flag (defined in search.h) is named "env_flag"
* The command flag argument (defined in command.h) is named "flag"

This convention is kept in functions like do_env_print(), do_env_set()
and do_env_delete(), but not in do_env_default().

Rename the hashtable flag in do_env_default() from "flag" to "env_flag".
Rename the command flag in do_env_default() from "__flag" to "flag".

No functional change.

Signed-off-by: Yaniv Levinsky <yaniv.levinsky@compulab.co.il>
Reviewed-by: Igor Grinberg <grinberg@compulab.co.il>