[settings] Allow "set" command to take an empty value

Allow "set <variable>" to be used to set the variable to an empty
value, if permitted by the setting type.  Note that some settings
backends do not differentiate between an empty value and a
non-existent value, so this may or may not be equivalent to "clear
<variable>".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/commands/nvo_cmd.c

 
 /** "set" command descriptor */
 static struct command_descriptor set_cmd =
-	COMMAND_DESC ( struct set_options, set_opts, 2, MAX_ARGUMENTS,
+	COMMAND_DESC ( struct set_options, set_opts, 1, MAX_ARGUMENTS,
 		       "<setting> <value>", "" );
 
 /**