Take a shot from each camera, sample each microphone for a few milliseconds, Sample the gyros and accelerometers for a few milliseconds. Sample the current battery voltage/charge state, Salt in the current time/date and last known location, along with the various readable serial numbers, SHA each of these sources and fold them into each other and SHA the result, and you should be good to go.
Once the device is booted, it can do a lengthy and more sophisticated RNG to make a seed that will be folded into the above entropy sources on the next boot.
But it does sound like apple should put in a good hardware entropy source on their A8 (and future) chips. (One with no NSA/CSEC/GCHQ/KGB/FSB backdoors please!)
That can also be folded into the above sources (I think you would never want to rely on only one source of entropy, no matter how good or trusted.)