A routine to conveniently read lines from TTY. Extracted from
shell. Will be used in other places.

That is, send current chr selection, and then, send LF.

This avoids having "ghost" chr selection sticking around.

This allows for outputting a character selection before it's given
through GetC. That allows a pad-oriented character selection scheme.

Still a bit rough around the edges, but still, works rather well.

This hook is called during every iteration, allowing regular screen
refresh when video is involved.

Now this is getting eminently exciting!

Make ASCII_BS (sent when driven through kbd) make the same thing as
ASCII_DEL. Also, don't crash on filling the buffer.

We'll make them uppercase when we'll add support for shift.

Also, add timer to reset reception status after 100us.

It previously returned success!

When there's a mismatch, retry up to a certain number of times.

This makes random problem related to assembling big kernels go away! But
it also make SD card reading much slower...

For now, this achieves nothing else than wasting cycles, but this is the
first step in enabling CRC verifications (CMD59).

I think that this is where my random problems with assembling large
kernels from SDC come from: bad data that isn't detected. If that
happens when PGM loads programs in memory, then anything can happen.

`sdct`, when ran often enough, will error out or corrupt away (go
crazy)...