Ethernet: Just detection eth%d

This will fix the problem that when android bootup with wired line
plug-in, it could not get ip address

Signed-off-by: LiuXin <liuxing@embedinfo.com>
set default dns to 8.8.8.8

when no dns was set, return the default dns 8.8.8.8 to application

Signed-off-by: Royce Jiang <jiangxd@embedinfo.com>