README.md: Update some notes about AMD CAR implementation

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
cpu/amd/agesa: Import AMD AGESA CAR implementation from coreboot

This imports the raw elements needed for a AMD AGESA CAR implementation
from Coreboot as-is, including any needed headers stripped down somewhat.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
cpu: Hook up CAR into bootblock and provide basic Ada entrypoint

See COMPILE notes ~ need a build system now..

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>