epicardium: implement FB API
Supersedes https://git.card10.badge.events.ccc.de/card10/firmware/merge_requests/57, by implementing the FB API only, deferring double-buffering for later.
Edited by q3k
Supersedes https://git.card10.badge.events.ccc.de/card10/firmware/merge_requests/57, by implementing the FB API only, deferring double-buffering for later.