CPU and CPU_MODEL variables must now be defined by `boards/**/Makefile.features` or 'cpu/CPU/Makefile.features' files instead of `**/Makefile.include. Currently blacklist the slwstk6000b that still define 'CPU_MODEL' in a file that is not a 'Makefile.features. This allows getting this sanity check script in master This allow using the variables when parsing dependencies.