.gitignore: Add build/cache/

Ignore this cache directory.

Signed-off-by: Angelika Tobisch <tobisch@dresearch-fe.de>
meta-hipos: add watchdog service to systemd

Watchdog works via drbcc and will be started at boot. It will then
be executed every 120s and delay reboot by 400s.

Signed-off-by: Angelika Tobisch <tobisch@dresearch-fe.de>