@ank - I have two cAP AC units that I have successfully flashed with yhudit's build from the hAP AC2 thread: Support for Mikrotik Hap AC2
Make sure to also install irqbalance as it improves download throughput from around 350mbps to 450+ mbps - see details in Blaze's post: Support for Mikrotik Hap AC2
Your steps are correct, not sure about any MTD files nor having to enable serial console, I didn't need anything like that.
Let me know if you need additional help getting the build to work on your cAP AC.
Also for anyone seeing this thread that might be asking themselves why someone would put an experimental version of OpenWRT instead of RouterOS, the answer in my case is pretty simple: support for 802.11r - it works extremely well with Apple iOS and makes roaming between APs seamless, as opposed to the disaster that is RouterOS in this regard.
I actually get better overall performance with this firmware on cAP AC than I get with the "official" OpenWRT for the wAP AC I have outside.