Revert "futility: Remove legacy bios.bin name"

This reverts commit 20fc7f70b1b4af046dac3eaa9ea777767a92b4a9.

Reason for revert: http://b/519562415

Original change's description:
> futility: Remove legacy bios.bin name
>
> Remove the legacy "bios.bin" name from the firmware updater output
> mode. Also remove the fallback check for "bios.bin" when building a
> manifest from a simple folder.
>
> BUG=b:517029427
> TEST=make runtests -j
> BRANCH=none
>
> Cq-Depend: chromium:7877775
> Change-Id: I04e8a7c4c3c1a186eaf02c204718b42b919835c9
> Signed-off-by: Yu-Ping Wu <yupingso@google.com>
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7883122
> Auto-Submit: Yu-Ping Wu <yupingso@chromium.org>
> Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
> Tested-by: Yu-Ping Wu <yupingso@chromium.org>
> Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>

BUG=b:517029427

Change-Id: I32ad1f01c9652818aaf5aafacfbd60fa49e0069c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/7895642
Commit-Queue: Renato Silva <rrsilva@google.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Renato Silva <rrsilva@google.com>
Owners-Override: Renato Silva <rrsilva@google.com>