Integration of Link Layer Security

This pull request addresses #526.

I added a new NETSTACK layer, called LLSEC, in between the MAC and the NETWORK layer for implementing link layer security. This change is in line with the current architecture and the handling of the LLSEC layer is similar to that of other NETSTACK layers. For example, link layer security can be disabled by configuring the nullsec driver.

The interface of LLSEC drivers comprises four functions `bootstrap`, `send`, `on_frame_created`, and `input`.  I have successfully used this abstraction for implementing two LLSEC drivers, which provide authentication, confidentiality, as well as replay protection. One of them uses a network-wide key (Noncoresec) and the other one uses pairwise keys (Coresec). This pull request only contains Noncoresec, which is an 802.15.4-compliant LLSEC driver.

The employed AES driver is encapsulated as a preprocessor variable. I included a software-based, as well as a hardware-accelerated AES driver. Implementing other such drivers should be straightforward using these examples.

To make all this work, I extended the framer to support frames with security headers. Furthermore, I adapted `contiki-sky-main.c` to support link layer security. I also refactored the CC2420 driver.

What is left is to adapt the main files of other platform's main files to support link layer security. However, this task can be deferred since everything continues to work when leaving link layer security disabled.

Further information is [here](https://github.com/kkrentz/contiki/wiki).