The log shows the NIC being detected by the boot scripts around 4.9 seconds into the boot. Status is toggled from DOWN to UP (briefly) to DOWN again and it doesn't go back to UP until 25-26 seconds. I've no idea why it's so slow, although we have seen Realtek chipsets start slowly on many machines so it may be something endemic in their drivers or hardware. It's possible to work around the problem by putting a sleep delay in the boot process. Create /storage/.config/autostart.sh and add the following:
It's a sucky workaround as your boot times are now 25 seconds longer, but XBMC will always start with the network (and thus the DB on the NAS) available. You can probably reduce the sleep value by a couple of seconds with experimentation.