Added the OpenMote platform and examples

I have copied the CC2538DK platform and examples and adapted it to the OpenMote platform, which is also based on the CC2538 SoC. The code compiles fine and I am able to upload it and run it on the target platform. Comments in the source code will need to be ammended to match the target platform, but this can be done in the future.
Great catch! Fixed in another pull request.

Speaking of this file, how about moving it into `cpu/cc2538`?