I'm not sure about openwrt, but I use Mikrotik/Routerboard products, and I can use either protocol. I connect the Arduino to the usb port on the Routerboard (gets power there too), and the ethernet shield to one of the ethernet ports. I can ssh into the router and connect to the Arduino usb like a serial monitor, or use the ip address with TCP/IP as either a server or client.
Maybe openwrt has that usb feature also.