Add an option to suppress the echo of modem.

In addition to the echo suppress, The following changes are also
included:
1) use readlines() instead of readline() to avoid some
buffered data like: ['\r\n', '+CEREG: 2\r\n', '\r\n', '+CEREG: 0\r\n']

2) Option disable_operation will put modem into a mode more silent.

3) Changing the logging level to extract more verbose information as we
still in the stage to understand the modem behavior.

BUG=chrome-os-partner:21514
TEST=Manually with https://gerrit-int.chromium.org/#/c/42272/

Change-Id: I5705d441a35a791b78d2d7998f6e2379ca5f444c
Reviewed-on: https://gerrit.chromium.org/gerrit/63898
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
Commit-Queue: Chun-Ta Lin <itspeter@google.com>
Tested-by: Chun-Ta Lin <itspeter@google.com>
1 file changed