How can I force boot from alternate drive with boot.config?

[Julian Elischer]

John Baldwin wrote:

> I think the unit number is largely ignored now.  The kernel used to believe it 
> for finding /, but the loader now reads /etc/fstab and sets a variable in kenv 
> to tell the kernel where to find /.

One of the most annoying "improvements" for the decade from my 
perspective..

because if / is not going to be where the loader thinks / is (because
that is a different F with a different /etc/fstab) then you need to
do lots of fancy footwork to undo the damage.

Oh and it's not terribly well documented how to get around it.
(that I've found)