[rf] Add common.py to client/cros/rf

rf_common.py seems to be a wrapper so that a common.py from another
directory can be imported.  common.py was meant to be copied into any
directory where it's needed, so let's just drop one into client/cros/rf
and adjust it to appropriately point at the client directory.

I've also sneaked in a change to import utils from client/common_lib
instead of client/bin as the functions being used come from
client/common_lib's utils anyway.

BUG=None
TEST=ran agilent_scpi_unittest.py
     grep shows no other uses of rf_common.py
     pylint on rf_utils.py

Change-Id: I7f92676a3c7c232ae335249662f6f40951f8b2d8
Reviewed-on: https://gerrit.chromium.org/gerrit/47305
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: David Schneider <dnschneid@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
4 files changed