systemd: Disable power key handling which would shut down the watch when pressed. MCE is there for that.

Make sure user services are only run as ceres. Since openssh uses pam login, ssh root@watchIp would open a new systemd user session and start asteroid-launcher or other services as root

systemd: gives right to /dev/mm_interlock and /dev/mm_v3d to the video group to allow to run asteroid-launcher as ceres on tetra

This reverts commit 04acc8b6.