BusyBox ifup udhcpc后台运行

/**********************************************************************
 *              BusyBox ifup udhcpc后台运行
 * 说明：
 *     发现ifup会调用udhcpc，但是并不会切换到后台运行，解决一下。
 *
 *                                  2016-10-11 深圳 南山平山村 曾剑锋
 *********************************************************************/

一、参考文档：
    . How to get BusyBox udhcpc to run in background forever?
        https://unix.stackexchange.com/questions/267733/how-to-get-busybox-udhcpc-to-run-in-background-forever

二、解决办法：
    . cat /etc/network/interfaces
        # interface file auto-generated by buildroot

        auto lo
        iface lo inet loopback

        auto eth0
        iface eth0 inet dhcp
            udhcpc_opts -b          # <------ 添加udhcpc参数

    . 启动输出信息：
        ...
        Starting network: eth0: Freescale FEC PHY driver [Generic PHY] (mii_bus:phy_addr=:, irq=-)
        udhcpc: started, v1.26.2
        udhcpc: sending discover
        PHY: : - Link is Up - /Full
        udhcpc: sending discover
        udhcpc: sending discover
        udhcpc: no lease, forking to background
        ...