Add TARGET_USERNAME variable to define username on target distro. Layer can be now used by others also.

user services: Rework the way ceres services are launched in a cleaner systemd-wide way. This also makes those recipes less asteroid-spcecific for systems with a different username.

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

buteo-syncfw: Start msyncd after booster-qt5-signal to make sure the first start doesn't stop in a SIGSEGV