EFM32 support

Support for the EFM32 Gecko and Giant Gecko processor families.
Adds a shared EFM32 library, as well as specific family implementations to the available processors, as well as the efm32g-st700 and efm32gg-st3700 evaluation boards.
Currently uses the Silicon labs CMSIS core and EMLIB peripheral interfaces rather than direct register operations, which should make updates fairly simple for new chips or peripherals. Swapping to register operations to remove EMLIB is not a huge amount of work if required.

Latest Travis build is here: https://travis-ci.org/ryankurte/RIOT/
Failing tests to resolve: 
- [ ] riot_and_cpp test
- [ ] bloom
- [ ] posix_semaphore
- [ ] pthread_rwlock
- [ ] struct_tm_utility
- [ ] thread_cooperation
Also not sure why other boards are failing things in my Travis build?

Todos:
- [ ] update new header blocks
- [ ] determine whether Silabs EMLIB licensing is okay for embedding in RIOT, or whether register operations are better
- [ ] squash commits

Still a bit of work to be done, but I thought this would be a good place to discuss and track it.
Anything else I have missed, or thoughts?

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