udhcpc client 不能正常工作，需要内核打开 packet socket 支持
Networking support -> Networking options -> Packet socket
说明
CONFIG_PACKET: │
│ │
│ The Packet protocol is used by applications which communicate │
│ directly with network devices without an intermediate network │
│ protocol implemented in the kernel, e.g. tcpdump. If you want them │
│ to work, choose Y. │
│ │
│ To compile this driver as a module, choose M here: the module will │
│ be called af_packet. │
│ │
│ If unsure, say Y.