The build system contains several instances of
INCLUDES += -I$(RIOTBASE)/sys/posix/include
This is bypassing the module management system, by directly accesing
headers without depending on a module. The module is the posix module.
That line is also added when one of the posix_* modules is requested.
According to the docs, the posix module provides headers only, but in
reality there is also inet.c.
This patch:
- Moves `inet.c` into `posix_inet`, leaving `posix` as a headers-only
module.
- Rename `posix` as `posix_headers` to make it clear the module only
includes headers.
- Makes `posix_*` modules depend on `posix_headers`, thus removing the
explicit `INCLUDES+=...` in `sys/Makefile.include`.
- Ocurrences of `INCLUDES+=...` are replaced by an explicit dependency
on `posix_headers`.
