ccd: Get the power button state from Cr50 console

In the lab, it checks the power button state to verify the health
between DUT and servo. Though, setting power button in Cr50 console
doesn't work on all platforms, getting its value is fine. So add
back its get method.

BUG=chrome-os-partner:62904
TEST=Started servod and run "dut-control pwr_button"
TEST=Started servod and calls the servod RPC calls:
>>> import xmlrpclib
>>> c = xmlrpclib.ServerProxy('http://localhost:9999')
>>> c.power_normal_press()

Change-Id: Iec59390ff750996846afb01bd452520efb19335c
Reviewed-on: https://chromium-review.googlesource.com/445961
Commit-Ready: Wai-Hong Tam <waihong@google.com>
Tested-by: Wai-Hong Tam <waihong@google.com>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: danny chan <dchan@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
3 files changed