stand: Add workaround for HP BIOS issues

hrs@ and kuriyama@ have found that on some HP BIOS, a system will fail to
boot immediately after installation with the claim that it can't work out
which disk they are booting from.

They tracked it down to a buffer overrun, and found that it could be
alleviated by doing a dummy read before-hand.

Submitted by: kuriyama
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14341