Fix for USB media not found at boot

[James R. Van Artsdalen]

Hans Petter Selasky wrote:
> The changeset in question has not been committed to mainstream.
>
> Scott Long is working on an improved solution. Please coordinate with him.

Has anything happened with the general case?  A friend of mine reported
to me a regression between 8.0-RELEASE and 8-STABLE that turns out to be
this issue with SATA port multipliers.

I have the same problem, except that I need to further wait for GEOM to
make visible a gmirror made of two slow devices (behind SATA port
multiplier).