[flashboot] GENERIC-RD kernel config change for acpi

[Josh "Xipher" Hoppes]

Hey, I was working on building a kernel to boot off a USB flash drive
and using the GENERIC-RD config from the snapshot it failed to build
with an error saying acpi's can not be on mainbus's, looking over the
man page I found that between 4.2 and 4.3 the i386 acpi device was
changed from "at mainbus?" to "at bios?". It's a quick one line fix in
the config but just in case some one has been trying to figure out why
their kernel won't build I thought I would point it out (didn't see any
mention of this in the archives).