MicroPython own readline implementation is superior now by providing
automatic indentation and completion (completion for GNU Readline was
never implemented). MICROPY_USE_READLINE=2 also wasn't build for a long
time and probably broken.
If GNU Readline is still beneficial for some cases, it can be achieved
with external wrappers like "rlwrap" (there will be the same level of
functionality, as again, there never was deep integration, like completion
support).
Avoid using system libraries, use copies bundled with MicroPython as
submodules (currently affects only libffi, other dependencies either
already used as bundled-only (axtls), or can't be bundled (so far),
like libjni).
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.
