busybox自带dhcp客户端和服务器，分别是udhcpc和udhcpd
修改内核配置：
Networking support-->
Networking opetions-->
<*> Packet socket
[*] TCP/IP networking
[*] IP: multicasting
[*] Network packet filtering framework (Netfilter) -->
否则会出现：
udhcpc: socket: Address family not supported by protocol
从busybox/examples/udhcp文件夹中，复制simple.script文件到开发板的/usr/share/udhcpc/目录下
运行udhcpc即可获取IP地址
