Look into CMake

[thiccaxe]

there are some previous issues that discussed CMake but it has not moved forward.

LLVM could be possible with tooling like https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm but this issue is for exploring is CMake will improve the currently messy makefile.

[djava]

This has been attempted 3 times in the past 4 years:

• feature/cmake
• feature/cmake-2
• feature/cmake-migration

Maybe some investigation into why it didn't make it across the line any of these times would be in order?

In particular, we'd have to migrate existing templates to CMake which may be difficult (templates can have their own custom .mk files, and at least a few of them actually do).

We also need motivation for this - while messy, the pros Makefile has proven to be remarkably resilient, and I don't remember any user issues that came down to a problem with Make or the Makefile.