aprinter-ramps13.cpp: Increase baud rate to 250000 and increase buffer
sizes.
It appears we can handle 250000 after having reordered the timers, and
that we still had enough RAM for the buffer size increment.
Try to detect stack overflow when running on AVR.
Periodically check whether stack overflow has changed the value of a
variable which (presumably) is located at the end of the data section.
Even if stack overflow causes the system to go haywire before this code
manages to detect the overflow, the watchdog timer should kick in.
So we can be reasonably confident that, as long as the printer is
working, there was no stack overflow.