Making select_eth_device work with 2nd NIC linked only and FETCH assigned.

Making select_eth_device work with 2nd NIC linked only and FETCH assigned.

The differences:

  1. If user uses "FETCH", "HTTPFS", or "FTPFS", select_eth_device
     should not exit since network is also required to get the
     filesystem.squashfs.

  2. Files in "/sys/class/net/" maybe not be up so quick, so it's
     better to use a while loop to wait for that.

  3. Another case is, for example, if eth0 and eth1 are linked, but
     only eth1 is able to get filesystem.squashfs, the original
     select_eth_device will try eth0 only. It's better to provide an
     alternative way to allow user to specify the device in boot
     parameter. Here in the patch live-netdev=eth1, for example, can
     be used to make it.