faft: Refactor Chrome EC related methods into a class

So far the Chrome EC related methods are quite distributed across tests.
We should have a single place to control all the Chrome EC functions.

This CL does the first step to refactor the UART command methods into
the ChromeEC class.

BUG=chromium-os:35254
TEST=run_remote_tests.sh --board link --remote dut suite:faft_ec

Change-Id: If148170bcf6801a2a33f666b8a5a7fadc47f3243
Reviewed-on: https://gerrit.chromium.org/gerrit/35403
Reviewed-by: Vic Yang <victoryang@chromium.org>
Reviewed-by: Mike Truty <truty@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
16 files changed