boards: add board full_name field

Full name or description of a board is something we are missing in
HWVv2. It is right now being added to yaml files parsed by twister. This
should be generically available to tooling and documentation
independently from twister.

As we rework how twister parses board meta-data (#77250) and how we
generate board documentation (#79160), this becomes neceassry.

Moving the board full name/description from the twister yaml files to
the board.yaml is something we can automate once the schema is agreed
upon.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>