If it helps any at all, we had this issue with our X250 after upgrading to 2012R2 SP1. Our smsts logs showed in Red not Yellow PxeGetPxeData failed with 0x80004005: also could not do ipconfig/renew which lead us to our boot.wim drivers
We tried several different drivers and even ones from Intel. Our working solution was to get the WinPE 5.0 drivers from Lenovo and update our boot wim with these. http://support.lenovo.com/us/en/downloads/ds102312
However, I did see a few posting on the Lenovo forums with the x240 having some issues such as these.
Our enterprise image is win7x64. I did add the win8.1 x64 driver to our boot wim and we are happy campers once again.
Good Luck!