servod: add command muxing in ec3po

This creates a second pty for pty_driver to open, to prevent the
need for freezing the user pty. This also allows the user pty to
fully log and print all output.

This also cleans up ec3po printouts and prints ptys by name.

BUG=b:74023102,b:73310923
TEST=servod is fast now, power_state:rec works now
CQ-DEPEND=CL:958128

Change-Id: I4a69456d91914393f2df26b13cc6d97fe7f21a79
Reviewed-on: https://chromium-review.googlesource.com/958127
Commit-Ready: Nick Sanders <nsanders@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Nick Sanders <nsanders@chromium.org>
4 files changed