Now that we have targets.json, we get target inheritance and can use it to clean up the EFM32 folder structure.
* In the top-level EFM32 folder, there are now folders per MCU family (Giant, Leopard, ...)
* Those family folders contain the CMSIS headers in the 'device' subfolder, as well as global family headers (i.e. mapping of pins to peripherals)
* Inside of the family folder, there is a per-target folder containing target settings. In the future, we'll want to get rid of those by using the config system provided by targets.json