Making a menu through serial
You could put your switch/case inside an infinite loop, and return from
the Menu() function when the user presses '3'. E.g.:

void Menu() {
    Serial.println(menu_banner);
    for (;;) {
        switch (Serial.read()) {
            case '1': ldrConfig(); break;
            case '2': srvConfig(); break;
            case '3': return;
            default: continue;  // includes the case 'no input'
        }
    }
}