Improve software UART for slow rates

[jfpoilpret]

Current software UART heavily uses ISR for receiving data.
Due to high bit rates, it has been decided that one ISR call should handle a complete byte, which is fast enough for high rates (e.g. 115'200 bps), but is terrible with slow rates (e.g. 9'600 bps) where a long time can be spent within an ISR, so that other interrupts will not get processed in time.
Typically, at 9'600 bps, each byte received will "block" the ISR during about 1ms.

Dealing with slow rates would be better with a Timer used as a clock for each bit reception.
This would not work at higher rates (the cost of calling an ISR for each bit is too heavy and we may miss some bits in this case).

Hence we propose to create a new kind of SW UART (and keep the existing one).