init: make xargs invocation busybox-compatible

busybox xargs does not have -d nor long --max-procs options, instead use -0 (and separate arguments with printf "%s\0") and -P which are more portable. While we are here, also add -r (--no-run-if-empty, which also has no long equivalent for busybox) as we likely don't want to run anything if no profile were found

This is useful for alpine systems where findutils is not installed by default, but busybox xargs is available.