there are two threads on this community about boot from SAN with UCS:
It shouldn't matter whether you're using Nexus 5K or attach the Nimble to the FIs.
However, there's something that changed (with 2.X firmware) since the two above were posted.
Where it says "obtain the target discovery IP" you now have to use the Data IPs of the array, instead of the arrays discovery IP.
Hope this helps.