RPi 3B Aduio 3.5mm output

/**********************************************************************
 *                RPi 3B Aduio 3.5mm output
 * 说明：
 *     看到树莓派上一个3.5mm耳机口，从来没用过，折腾一下。
 *
 *                                2018-7-13 深圳 宝安西乡 曾剑锋
 *********************************************************************/

一、参考文档：
    1. 树莓派如何选择3.5mm耳机孔输出声音
        https://jingyan.baidu.com/article/64d05a02220053de55f73bbc.html

二、具体操作：
    1. sudo raspi-config
    2. 选择流程：
        Raspberry Pi 3 Model B Rev 1.2
        
        ┌─────────┤ Raspberry Pi Software Configuration Tool (raspi-config) ├──────────┐
        │                                                                              │
        │    1 Change User Password           Change password for the current u        │
        │    2 Network Options                Configure network settings               │
        │    3 Boot Options                   Configure options for start-up           │
        │    4 Localisation Options           Set up language and regional sett        │
        │    5 Interfacing Options            Configure connections to peripher        │
        │    6 Overclock                      Configure overclocking for your P        │
        │    7 Advanced Options               Configure advanced settings          <---│
        │    8 Update                         Update this tool to the latest ve        │
        │    9 About raspi-config             Information about this configurat        │
        │                                                                              │
        │                                                                              │
        │                                                                              │
        │                     <Select>                     <Finish>                    │
        │                                                                              │
        └──────────────────────────────────────────────────────────────────────────────┘
        ┌─────────┤ Raspberry Pi Software Configuration Tool (raspi-config) ├──────────┐
        │                                                                              │
        │    A1 Expand Filesystem             Ensures that all of the SD card s        │
        │    A2 Overscan                      You may need to configure oversca        │
        │    A3 Memory Split                  Change the amount of memory made         │
        │    A4 Audio                         Force audio out through HDMI or 3    <---│
        │    A5 Resolution                    Set a specific screen resolution         │
        │    A6 Pixel Doubling                Enable/Disable 2x2 pixel mapping         │
        │    A7 GL Driver                     Enable/Disable experimental deskt        │
        │                                                                              │
        │                                                                              │
        │                                                                              │
        │                                                                              │
        │                                                                              │
        │                     <Select>                     <Back>                      │
        │                                                                              │
        └──────────────────────────────────────────────────────────────────────────────┘
                  ┌──────────────────────────────────────────────────────────┐
                  │ Choose the audio output                                  │
                  │                                                          │
                  │             0 Auto                                       │
                  │             1 Force 3.5mm ('headphone') jack     <---    │
                  │             2 Force HDMI                                 │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │                                                          │
                  │              <Ok>                  <Cancel>              │
                  │                                                          │
                  └──────────────────────────────────────────────────────────┘
        
        
    3. speaker-test -t sine