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.