futility: updater: Add '--servo_port' to select from multiple servods

When multiple servo boards are connected to the host, we usually want to
select the right servo by specifying its port, get the servo serial
number, and pass that to flashrom programmer.

The new --servo_port (or environment variable SERVOD_PORT) now allows
developers to flash firmware via specific servod using futility updater

 futility update --servo_port 9998 -i PATH_TO/image.bin

TEST=make runtest; sudo futility update --servo_port 9998 -i image.bin

Change-Id: Ic302f841abf745801995ff233fc209726ed039c8
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2228258
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Douglas Anderson <dianders@chromium.org>
3 files changed