I am not sure if this happens under all circumstances but I wasn't able to use the boot.wim file from a Server 2012 R2 setup media. In my WDS server, I added a boot.wim file from the Server 2012 R2 setup ISO as a new Boot Image. I used a vm to boot into the PXE server and try the new boot image but I got the following error;
Status : 0xc000000f
Info: The selected entry could not be loaded because the application is missing or corrupt.
This is the result I was getting from an untouched boot.wim. I never tried to create a seperate WinPE media to see if that would work but I found this on a Spiceworks forum thread. (btw, the SpiceWorks community is awesome!).
A forum member called John Masson sent this solution below and it's pretty solid. I've tested this solution in my lab.
It will sound stupid and non-sense but this is actually working :)
Basically, you have to mount the boot wim file and unmount it while using the "commit" switch even if we are not making any changes. I am just pasting the solution here;
Mount the WIM file with the command
dism /mount-wim /wimfile: /mountdir: /index:1
Unmount the WIM file, committing changes (even though you didn't make any) with the command
dism /unmount-wim /mountdir: /commit
The only thing I can add to that the WIM you need to modify is normally located under the default WDS folder ".\RemoteInstall\Boot\x64\Images\boot.wim".