Enable UART capturing on EC interface

We want to enable the UART capturing on EC interface such that we can record
the UART output for the debug purpose.

BUG=chrome-os-partner:19288
TEST=Manaul
On Link, set 'dut-control ec_uart_capture:on'
and check 'dut-control ec_uart_stream' output the EC UART results.
More complete tests refer to the FAFT change.

Change-Id: I9f4b9a409d1763eae99b9507384d2ab6516fb99f
Reviewed-on: https://gerrit.chromium.org/gerrit/55762
Reviewed-by: Yusuf Mohsinally <mohsinally@chromium.org>
Commit-Queue: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
3 files changed