CPU must be defined by `$(RIOTBOARD)/$(BOARD)/Makefile.features` or
one of its common included Makefile.features file.
This currently only adds a warning but not prevents building for the
moment.
Prepare for when boards define `CPU` in `BOARD/Makefile.features`.
Include '$(RIOTCPU)/$(CPU)/Makefile.features' directly when it is
defined. This will allow removing the file inclusion from the
`BOARD/Makefile.features`. The board must then not include it directly.
Transitional change to allow migrating part by parts.
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
Put the definition of `FEATURES_USED` in common and use the variable
instead of duplicating code.
This required defining 'FEATURES_OPTIONAL_ONLY|USED' to not overwrite
the value of 'FEATURES_OPTIONAL' as was done before.
Also add 'FEATURES_OPTIONAL_MISSING' to list optional features that were
not included as not provided.
This removes the need to print FEATURES_MISSING with the optional
features too.
This will allow sharing it between Makefile.include and
makefiles/info-global.inc.mk.
Also some common variables definition can also be moved to here.
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
