wardmodem: Implement multiplexed serial channel end point.

- Implement serial end-point with read/write at AT command granularity.
- Implement a multiplexer that can send data from modem manager to both the
  physical modem and wardmodem, and verify the responses.

TEST=run unit-tests.
(1) python at_channel_unittest.py
(2) python at_transceiver_unittest.py

Change-Id: I59a4eefc8708fb418cede4a67adaa7904ebd82b0
Reviewed-on: https://gerrit.chromium.org/gerrit/57533
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
4 files changed