Added the OpenMote platform and examples / Do NOT Merge

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.
Hi @peretuset , 

I think this line can be removed due to the antenna_init() function already selects the external antenna by default. You might remove this line or remove the selection of the external antenna from the antenna_init() and let this line as it is.