experimental: move platform preprocessor definitions to cmake

[antoinevg]

This PR moves the platform definitions (e.g. IS_PRALINE, IS_HACKRF_ONE etc.) into cmake.

The specific issues it addresses are:

• The include order of platform_detect.h matters whenever #ifdef IS_<some_board> is used. If it is not included first it can result in compile errors that are not immediately obvious.
• This causes false negatives with include-what-you-use resulting in a sprinkling of IWYU pragma: keep across the sources.

These have caused some friction for me recently.

The downsides are:

• Moving things into cmake.
• It uses add_compile_definitions() which is only available in cmake >= 3.12. Though it could be implemented using add_definitions() which would just look uglier.
• switch (detected_platform()) can no longer evaluate that a given check is exhaustive. That said, we don't have too many of those left.
• The optional introduction of IS_UNIVERSAL (see 3rd commit). An argument can be made that this helps to annotate portions of the code that are affected when building a universal binary but it's been nice to not have it so far.

The main advantage I see is that it generally makes the usage of platform_detect.h a little less fraught.

Would love to hear feedback!