Comment 7 for bug 1239820

As I've mentioned before, a recurring theme in the logs you have linked:

http://validation.linaro.org/scheduler/job/79440/log_file#L_15_435
http://validation.linaro.org/scheduler/job/79424/log_file#L_31_367
http://validation.linaro.org/scheduler/job/78934/log_file#L_15_367
http://validation.linaro.org/scheduler/job/78926/log_file#L_29_366

is that udhcpc is already well into it's DHCP requests before the kernel has even noticed that the network link is up, so a considerable amount of the total time udhcpc is willing to wait, is lost without any requests making it out of the board.
The DHCP server then only has a short amount of time to respond. On a smaller network that is probably fine, but with the large amount of devices in our and the sheer amount of DHCP traffic it's likely taking just a tiny bit too long to respond and the board has already given up.