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.
41d10cf005