[linprocfs] add sys/kernel/random/boot_id

This file holds a UUID that is invariant for every boot. In our case, it is
invariant per mount.

PR: 262369
Reported by: sodynet1@gmail.com
Approved by: dchagin@
Differential Revision: https://reviews.freebsd.org/D34860

(cherry picked from commit 0f2041a5b03259a8a7b20ffc98e34884cc5590b3)