Ich zitiere nicht gerne Manpages oder verweise stumpf darauf, da ich das unhöflich finde. Auch ich hab es oft, dass ich gar nicht weiss wo oder nach was ich suchen soll oder einfach überfordert bin vor Informationen. Dennoch beantwortet loader(8) die Frage nach den Variablen sehr gut:
boot_mute
All console output is suppressed when console is muted. In a
running system, the state of console muting can be manipulated
by the conscontrol(8) utility.
boot_verbose
Setting this variable causes extra debugging information to be
printed by the kernel during the boot phase.
autoboot_delay
Number of seconds autoboot will wait before booting. If this
variable is not defined, autoboot will default to 10 seconds.
If set to “NO”, no autoboot will be automatically attempted
after processing /boot/loader.rc, though explicit autoboot's
will be processed normally, defaulting to 10 seconds delay.
If set to “0”, no delay will be inserted, but user still will
be able to interrupt autoboot process and escape into the
interactive mode by pressing some key on the console while
kernel and modules are being loaded.
If set to “-1”, no delay will be inserted and loader will
engage interactive mode only if autoboot has failed for some
reason.