Complete the twi/i2c hal

[jnaulet]

As i2c is probably my least favorite peripheral to develop and test, a lot has been left out.

It requires at least one more interface in the form of twi_poll() for the slave part.

A simple not twi_poll(struct twi *ctx) that returns -EAGAIN, 0 (write) or 1 (read) should do the trick.

This will allow a slave to know if he’s addressed for a read or a write, allowing asynchronous read and writes.

this is already present in different state machines so it’s just moving code around, the big part of the work will be to re-test everything.

Regarding the setup, 10bit mode is missing, unbalanced scl hi/Lo is not supported, etc, etc and most of the drivers simply ignore basic errors like arbitration loss.