I encountered this problem too. What I did to rollback to the 08-2014 firmware was booting the device with the last RootFS and Kernel by pressing (power+x+y) and install the previous update from May 2014, after the the device has that firmware and you can install the 08-2014 update.
I tried to install the 08-2014 firmware with the old kernel(08-2014) and old rootFS but only the rootFS will be installed and the it will still load the kernel from the nightly build by default which will result in a broken state, in this case you just need to load with old kernel by pressing (x+power). I think the system won't install the same kernel twice(makes sense!) so to overwrite the new kernel you need to install an older kernel.
I hope this can help others who wants to rollback from the nightly builds.
Edit: this work only if your last working kernel and rootFS is from the 08-2014 like in my case else you need flash the device