firmware_SelfSignedBoot: Add servo USB media auto-detect on dut.

Previously, servo USB device was hard-coded based upon CPU architecture.
Change the test to do autodetection.

BUG=chrome-os-partner:21171.
TEST=Run firmware_SelfSignedBoot on peppy, verify pass.

Change-Id: Iad8735bccf5f9a36c7c86a3fa2c9bf3136f5e03d
Reviewed-on: https://gerrit.chromium.org/gerrit/63790
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Commit-Queue: Shawn Nematbakhsh <shawnn@chromium.org>
Tested-by: Shawn Nematbakhsh <shawnn@chromium.org>
1 file changed