Cleanup of boards and cpus makefiles.

[cladmi]

Description

In my goal to document the build system. I spent some time looking at boards and cpu Makefiles.
I found some inconsistencies and now useless configurations.

Expected results

I would like to remove these inconsistencies to have cleaner and more consistent configurations between boards/cpus.
This should also simplify the task of documenting them.

In a short term, it only refactors makefiles without changing the functionality.
But for a long term goal, it should cleanup and refine build system concepts which would help define requirements for build system upgrades.

Detailed steps

This issue will list the problem found over time and link to dedicated PRs:

• Native is using a special NATIVEINCLUDES: native: remove non required NATIVEINCLUDES #8652 (maybe it can be removed)
  • Cannot be removed
  • Add a documentation about it
  • Re-order includes to match RIOT include order
  • Also, I would like to check if it would make sense and would be possible to only use system posix headers in native. My reason is that if a module using RIOT posix headers, communicates posix values with a module using NATIVEINCLUDES, then they would understand it differently.
• Inconsistent include order between cpu, boards and common. Makefile.include: fix board/cpu inconsistent include order #8717
• Duplicate includes of $(CPU)/include and $(BOARD)/include
• Repetitions required when using 'common' cpus and boards.
• Dependencies declared in board/cpu include.
  • Makefile.dep: process cpu dependencies and fix stm32_common Makefile.dep: process cpu dependencies and fix stm32_common #9892
  • Start moving dependencies
• native defines CC, LINK, CXX etc even though they are already defined by gnu.inc.mk, llvm.inc.mk boards/native: remove toolchain variables duplication #11627

[jnohlgard]

One more point: native defines CC, LINK, CXX etc even though they are already defined by gnu.inc.mk, llvm.inc.mk