[settings] Prefill existing setting value in "read" command

When prompting the user to enter a setting value via the "read"
command, prefill the input buffer with the setting's current value.

Requested-by: Ján Ondrej (SAL) <ondrejj@salstar.ks>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/commands/nvo_cmd.c

  */
 static int set_core_exec ( int argc, char **argv,
 			   struct command_descriptor *cmd,
-			   int ( * get_value ) ( char **args, char **value ) ) {
+			   int ( * get_value ) ( const char *name,
+						 char **args, char **value ) ) {
 	struct set_core_options opts;
 	const char *name;
 	char *value;
 	name = argv[optind];
 
 	/* Parse setting value */
-	if ( ( rc = get_value ( &argv[ optind + 1 ], &value ) ) != 0 )
+	if ( ( rc = get_value ( name, &argv[ optind + 1 ], &value ) ) != 0 )
 		goto err_get_value;
 
 	/* Determine total length of command line */
 /**
  * Get setting value for "set" command
  *
+ * @v name		Setting name
  * @v args		Remaining arguments
  * @ret value		Setting value
  * @ret rc		Return status code
  */
-static int set_value ( char **args, char **value ) {
+static int set_value ( const char *name __unused, char **args, char **value ) {
 
 	*value = concat_args ( args );
 	if ( ! *value )
 /**
  * Get setting value for "clear" command
  *
+ * @v name		Setting name
  * @v args		Remaining arguments
  * @ret value		Setting value
  * @ret rc		Return status code
  */
-static int clear_value ( char **args __unused, char **value ) {
+static int clear_value ( const char *name __unused, char **args __unused,
+			 char **value ) {
 
 	*value = NULL;
 	return 0;
 /**
  * Get setting value for "read" command
  *
+ * @v name		Setting name
+ * @v args		Remaining arguments
  * @ret value		Setting value
  * @ret rc		Return status code
  */
-static int read_value ( char **args __unused, char **value ) {
+static int read_value ( const char *name, char **args __unused, char **value ) {
+	char *existing;
+	int rc;
 
-	*value = readline ( NULL );
-	if ( ! *value )
-		return -ENOMEM;
+	/* Read existing value */
+	if ( ( rc = fetchf_named_setting_copy ( name, &existing ) ) < 0 )
+		return rc;
+
+	/* Read new value */
+	*value = readline_history ( NULL, existing, NULL );
+
+	/* Free existing value */
+	free ( existing );
 
 	return 0;
 }