Add support for TL-WR710N

The device (EU version) has 8MB flash: http://wiki.openwrt.org/toh/tp-link/tl-wr710n. A bunch of devices will be deployed at the Teufelsberg site.

@db4um: can you test [this build](http://buildbot.berlin.freifunk.net/buildbot/unstable/ar71xx/377/)?