servod: provide ability to retrieve servo serial number

It is useful to be able to find out the serial number of the servo
servod is connected to. Add a 'pseudo control' to retrieve the serial
number, if known by servod.

BRANCH=none
BUG=none
TEST=ran the following commands:
    $ dut-control serialname
   serialname:780735-00084
   $ dut-control -i serialname
   serialname:serial number of the connected servo

Change-Id: Ib5ed09ece69dd104d0692b9794f01456facd6092
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/262204
Reviewed-by: Todd Broch <tbroch@chromium.org>
2 files changed