Add the latest 2.12.x upstream stable branch.
Both 2.4.x and 2.12.x can live side-by-side, however only the latest
stable branch/releases are security-maintained, so add it unslotted.
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
