I have a VIA M'SERV S2100 with FreeNAS 8.
Since I upgraded from FreeNAS 7, the M'SERV no longer turns back on after suffering a power failure. As I remember it, there was a setting for it in the old FreeNAS control panel, but I haven't been able to find one in the new one.
So logically, it should be something I could set from the CLI or some config file. Can someone point me in the right direction?
Like Tom said this is usually a BIOS setting. As far as I know there's nothing in stock FreeBSD that lets you tweak this though there may be something suitable in the ports tree.
The OS knows nothing until the system is turned on and the boot process starts, so BIOS/EFI/Whatever needs to take care of the turning-on-the-power part first.