v2.3.2: CMake and PlatformIO support

This is a maintenance release that should be fully compatible with existing code:

• Added support for CMake (see Build System CMake)
• Added support for PlatformIO (see Using PlatformIO)
• Upgrade to LMIC release 2.3.2 (see https://github.com/mcci-catena/arduino-lmic/releases)