Are you creating the kext instead of kernelcache? If you do this things from OS 10.10 single mode user you must start with kext-dev-mode=1
Here's what I've been trying to do:
I can't do anything in the single user mode of 10.10 because the PS2 keyboard drivers are not being loaded and I have no USB keyboard available right now, so I'm trying to generate kernelcache for 10.10 whilst on my 10.9 system, using the instructions you have written. But, as said before, it omits FakeSMC and VoodooPS2Controller kexts due to lack of proper signing apparently (I use RehabMan's FakeSMC fork for ProBooks).
Is the file present in /Volumes/Install/System/Library/Caches/com.apple.kext.cache/Startup/kernelcache ?
If the answer is yes you can launch the installation with kext-dev-mode=1 -v -s and the rebuild the kernel cache, with the same command, from there.
If the answer is no I think you must add the kexts to AppleKextExcludeList.kext
How can I view the contents of kernelcache file? Also, I've tried adding those kexts to the excludelist using KextExcludeListUpdater (sudo KextExcludeListUpdater -f /Volumes/Install/System/Library/Extensions/VoodooPS2Controller.kext -k /Volumes/Install/System/Library/Extensions/AppleKextExcludeList.kext - is this the right syntax?) but it didn't make any difference.