command-backspace will send $08 (as will Control-H).
If you have an extended keyboard, delete will also send $08.
(mame uses delete to toggle keyboard mode so it's not yet tested).
this also adds a hexdump in the CDA for the outgoing buffer
in local mode, "incoming" data is read from the outgoing buffer.
Currently data is (potentially) sent on each run of the main loop.
I thought about using the TX empty interrupt but it still needs to be
kicked off at some point (unlike incoming data)