login : function(component) {
    // username and password passed from component
    var usernameValue = component.find("username");
    var passwordValue = component.find("password");

    // invocation of server-side login function
    var usernameFound = component.get("c.lightningLogin2");
    usernameFound.setParams({
        username: usernameValue,
        password: passwordValue
    });
}

@AuraEnabled
public static void lightningLogin2 (String username, String password) {
    User u = Plexus_Utility.getUserDetails(username);
    if (u != null) {
        Site.login(username, password, '/s');
    }
}