(build) Restructured build system for better modularity

  * Added `cmake/Platform.cmake` for platform-specific tweaks;
    currently includes tweaks for the Raspberry Pi (currently
    by looking for "Raspian" in /etc/issue)

  * Added configuration (cmake) options to disable comms and
    input builds.

  * On Linux, we automatically disable building the input
    library if libevdev isn't available.
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/CMakeLists.txt