The stdio implementation of the standalone greentea-client wasn't
streaming "KiVi protocol" messages through the serial IO. Separate out
the existing implementation for greentea-client that uses
mbed_retarget.h's `read` and `write` functions so greentea tests
continue to work with the standalone client.

The IO fixture is shared between the "legacy" embedded greentea-client
and the newer standalone client.