Update the FEATURES_OPTIONAL meaning to be more in line since
FEATURES_USED is defined. Handle FEATURES_OPTIONAL as a configuration from
the BSP/build that should not be changed anymore after.
`FEATURES_OPTIONAL` are by definition optional so are not supposed to
cause a build to fail.
Only the 'REQUIRED' ones that are not 'PROVIDED' are 'MISSING'.
* Do not change FEATURES_OPTIONAL to remove REQUIRED features
* Prepare for having a different variable for the previous value
* Update dependency resolution/info-build as FEATURES_OPTIONAL cannot be missing
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.
