According to source on GitHub, it appears support for Commodore 64 and 128 platforms is part of the 3.0 release.
https://github.com/contiki-os/contiki/tree/3.0/platform/c64
https://github.com/contiki-os/contiki/tree/3.0/platform/c128
https://github.com/contiki-os/contiki/tree/3.0/platform/c64
https://github.com/contiki-os/contiki/tree/3.0/platform/c128