This re-adds `RIOT_MCU` as alias for `RIOT_CPU` and marks it as
deprecated. That should make life easier for downstream apps that may
still use `RIOT_CPU`.
Adding this macro in the definition of a macro causes a warning about
the deprecation to be emitted when used (and a build failure with
`WERROR=1`). This is useful as no other means to deprecate preprocessor
macros are provided.
The macro will be defined empty for compilers that are not GCC or
clang.
This adds a `riotbuild-prefix.h` that is added to the `riotbuild.h`
and processed by Doxygen. It solves two problems:
1. The pre-defined macros where previously fully undocumented, but
may be useful to real world applications
2. It provides a place where backward compatibility aliases can be
added with a deprecation notice
