cros_bundle/write_firmware: Support --servo parameter

Allow a selected servo port to be specified with --servo.

BUG=chromium-os:34729
TEST=manual
Try different options manually to make sure they work:

My snow servo is on 9999, link on 8888. Check that everything works as
expected:

Snow:

(cros) $ cros_bundle_firmware -b daisy --servo 9999 -w usb

Snow: (which is the default on my machine)

(cros) $ cros_bundle_firmware -b daisy --servo any -w usb

Gives an error:
raise IOError('No servo access available, please use --servo')
(cros) $ cros_bundle_firmware -b daisy --servo none -w usb

Link:
(cros) $ cros_bundle_firmware -b link --servo 8888 -w em100

Change-Id: Iefbf1c33e96cda777ca80253d287c1944f11b7ec
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/33970
Reviewed-by: Randall Spangler <rspangler@chromium.org>
2 files changed