Connected as described at http://printrbot.com/shop/led-strip/

Based on patch by Kelly Anderson <kelly@xilka.com> at
http://www.xilka.com/printrbot/marlin/1.1.4/20170707/

Without this...  The LSF won't work because none of the sampled points
in this code block get added.

We can save more and a pile of RAM by eleminating the eqnBVector and
EqnAMatrix arrays next.

Fixes #7250

Addressing #7227

Sends pre-defined string as action command when printer is kill.

Probing with the effector in the printing area, but an eccentric probe (e.g. allen key) outside it but still touching the bed gives meaninfull information for calibration. Since calibration is most accurate when probing as close to the towers as possible the testing was way to restrictive hence this fix.

Allows the user to send text to the serial console in order to
communicate with a host - sending debuging information or action
commands, for example.  Text must begin with '//' and this is added if
it is not already present at the beginning of the string.

Can't compile with PINS_DEBUGING enabled on Atmega1284p because it tries
to enumerate pins 46 and 47 (which don't exist) because `#ifdef
AVR_AT90USB1286_FAMILY ` doesn't work as expected.

Addressing #6992 and #5851