add default-option to bootmenu

with this patch the selected Entry in bootmenu can be set by
environment-var bootmenu_default=<number>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>

cmd/bootmenu.c

@@ -253,6 +253,7 @@ static struct bootmenu_data *bootmenu_create(int delay)
 
 	int len;
 	char *sep;
+	char *default_str;
 	struct bootmenu_entry *entry;
 
 	menu = malloc(sizeof(struct bootmenu_data));
@@ -263,6 +264,10 @@ static struct bootmenu_data *bootmenu_create(int delay)
 	menu->active = 0;
 	menu->first = NULL;
 
+	default_str = env_get("bootmenu_default");
+	if (default_str)
+		menu->active = (int)simple_strtol(default_str, NULL, 10);
+
 	while ((option = bootmenu_getoption(i))) {
 		sep = strchr(option, '=');
 		if (!sep) {