config: backups must be allowed on cdroms as well

This allows to pull a configuration from the installed system
via bsdinstaller and then either boot into the LiveCD or install
the system again.  Especially useful in an upgrade scenario so
that the system is cleaned but all settings are preserved.

etc/inc/config.lib.inc

@@ -815,9 +815,6 @@ function get_backups() {
 function backup_config() {
 	global $config, $g;
 
-	if($g['platform'] == "cdrom")
-		return;
-
 	conf_mount_rw();
 
 	/* Create backup directory if needed */