Do not remove the '-D' and '-U' values from CFLAGS.
This prevents issues where a '-D' could contain a space.
Some values way be duplicated from the 'riotbuild.h' header and the
command line but with the same value so without conflict.
To not put too many things in the command line, the -DMODULE_NAME are
only put in CFLAGS_WITH_MACROS.
Also, as now, the deferred value of CFLAGS is used for 'riotbuild.h',
macros set after the inclusion of `Makefile.include` will be taken into
account.
Refactor to define REALMODULES incrementally without overwriting
'NO_PSEUDOMODULES'.
This also allows an external makefile to maybe add something there if it really
needs to.
Factorize the reused value in a private variable.
I define it private as somehow 'USEPKG' is supposed to be removed so the
variables can be removed later.
BASELIBS is used only in the main Makefile.include or included files.
As it is not used in sub make executions or scripts it does not need to
be exported.
`#ifdef FEATURE_PERIPH_NAME` usage should now be replaced by
`#ifdef MODULE_PERIPH_NAME`.
This deprecation was started by pull requests #8226 and #8227.
