This gets uPy readline working with unix port, with tab completion and
history. GNU readline is still supported, configure using
MICROPY_USE_READLINE variable.
Also provides setraw() function from "tty" module (which in CPython is
implemented in Python). The idea here is that 95% of "termios" module usage
is to set raw mode to allow access to normal serial devices. Then, instead
of exporting gazillion termios symbols, it's better to implement it in C,
and export minimal number of symbols (mostly baud rates and drain values).
ffi is needed to use micropython-lib, so let's have it enabled by default,
then folks who have troubles with libffi can disable it, instead of everyone
doing manual actions again and again.
