API proposal for AES and hardware random number generator

This isn't really a real pull request in the sense that I want it merged, I just wanted to share what I have with everyone to get some feedback on the appropriate way to abstract away things like this.

Ultimately we need a way to abstract away AES block operations (and potentially CBC-MAC) and random-number generation. However, probably want a AES-CCM+ implementation in the contiki tree (what would use the abstracted AES block operations). We also may want to have a contiki-specified whitening function for whitening raw entropy generated from hardware, because this is something that is often skipped and can significantly affect the resulting security of the device.