Some radio chips have memory-mapped regions for transmit and receive buffers.
This change allows the radio driver to pass those buffers, when available,
directly to upper layers and avoid unnecessary copies.
This was originally here because of some misconfigured TTY settings,
and can cause problems when running on embedded devices.
Since the misconfigured TTY settings have been addressed, this check
is no longer needed.
* posix: logging: Use `stderr` instead of `stdout` for debug logging.
* posix: serial: Restore socket `termios` settings for `stdin` and `stdout` at exit.
Jonathan Hui