Use correct board name instead of 'generic' for OStree

Changed naming scheme for OStree repository and branch.
Now using board name instead of hardcoded 'generic' board.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D7386