prison0's hostuuid will get set by the hostid rc script, either after generating it and saving it to /etc/hostid or by simply reading /etc/hostid.
Some things (e.g. arbitrary MAC address generation) may use the hostuuid as a factor in early boot, so providing a way to read /etc/hostid (if it's available) and using it before userland starts up is desirable. The code is written such that the preload doesn't *have* to be /etc/hostid, thus not assuming that there will be newline at the end of the buffer or even the exact shape of the newline. White trailing whitespace/non-printables trimmed, the result will be validated as a valid uuid before it's used for early boot purposes.
It doesn't seem necessary at this time to be concerned with kern.hostid.