For EBB - add Abort command - to stop long moves

Once we add 3 byte long step and duration parameters for the SM command, and 
add commands for homing, we start to see some things take very long times (a 
3-byte duration in milliseconds is 4.7 hours). If something goes wrong, or you 
need to bail on a long command, it would be very useful to have an 'abort' 
command that would allow you to stop a long move or a home operation.

Note that, with the current system of flow-control built into USB, if you send 
two long SM commands back to back, the abort won't do you any good because it 
can't be processed until after the second long move is processed. 

However, if you're careful, just send out one long move at a time, then you can 
abort.

In the future, we can make a more sophisticated flow control method (where 
there is a longer FIFO of queued moves in the EBB, and the USB interface never 
blocks) or something like that, and then the abort will have more meaning.

We could also use a digital input as an abort. Right now, the PRG button acts 
as a 'pause' button, but this is something handled up at the application level 
(Inkscape) rather than at the EBB level. Having a hardware Estop input might be 
really nice.

Original issue reported on code.google.com by brian.schmalz on 29 Apr 2014 at 10:03

corba22 / eggbotcode

For EBB - add Abort command - to stop long moves #80