Fix for #1460.

Seems to work with only one extruder. Can't test this myself with more.

Z_DUAL_ENDSTOPS is a feature to enable the use of 2 endstops for both Z
steppers - Let's call them Z stepper and Z2 stepper.
That way the machine is capable to align the bed during home, since both
Z steppers are homed.
There is also an implementation of M666 (software endstops adjustment)
to this feature.
After Z homing, this adjustment is applied to just one of the steppers
in order to align the bed.
One just need to home the Z axis and measure the distance difference
between both Z axis and apply the math: Z adjust = Z - Z2.
If the Z stepper axis is closer to the bed, the measure Z > Z2 (yes, it
is.. think about it) and the Z adjust would be positive.
Play a little bit with small adjustments (0.5mm) and check the
behaviour.
The M119 (endstops report) will start reporting the Z2 Endstop as well.

- Address #1262 by leaving values as set
- Rename `add_homing` to `home_offset`

- Use named axis indexes, `X_AXIS` etc.
- Replace `block.steps_A` with block.steps[A]`
- Replace `A_segment_time` with `segment_time[A]`
- Add `A_AXIS`, `B_AXIS` for `COREXY` axes
- Conditional compile based on `EXTRUDERS`
- Add BLOCK_MOD macro for planner block indexes
- Apply coding standards to `planner.h` and `planner.cpp`
- Small optimizations of planner code
- Update `stepper.cpp` for new `block` struct
- Replace `memcpy` with loops, let the compiler unroll them
- Make `movesplanned` into an inline function

Warning: suggest parentheses around comparison in operand of &

pretty much helped.

- Add BIT and TEST macros
- Add _APPLY_ macros to stepper.cpp to help with consolidation
- Consolidate code in stepper.cpp using macros
- Apply standards in stepper.cpp
- Use >= 0 instead of > -1 as a better semantic
- Replace DUAL_Y_CARRIAGE with Y_DUAL_STEPPER_DRIVERS

With this change a mechanical or optical switch may be used to check the
availability of the filament and when the filament runs out an M600
(filament change) command is issued. This is only done while printing
with an SD card.

This feature was requested several times (issue #679), but the requests
were not accepted since it was believed that this situation should be
handled at host side. However during an SD print the control is totally
on firmware and I think that during an SD print it should be handled by
the firmware.

The original code was posted at reprap forum
(http://forums.reprap.org/read.php?1,297350) by Lazymonk. I have only
corrected some bugs of the code and improved it by adding definitions to
the configuration.h in order to make it more standardized.

- Nonlinear auto bed leveling code (includes G29, G30, Z_RAISE_AFTER_PROBING). Cleaned it up to be a delta-specific AUTO_BED_LEVELING_GRID code path.
- Allen key z-probe deployment and retraction code. Cleaned it up and added safety checks.

- Moved sanity-checks to Marlin_main.cpp
- Applied to other configuration files
- Fixed formatting of ABL output
- Passing verbose level to probe_pt
- Miscellaneous cleanup
- Put CONFIG_STEPPERS_TOSHIBA into Configuration.h

The X_Axis could not home to min while Y_Max endstop was trigged.

- Also adds a language string for the error.
- Also adds SERIAL_EOL as an alias for SERIAL_ECHOLN(“”)

Also fix analogInputToDigitalPin so it uses proper C hex notation

Also fix analogInputToDigitalPin so it uses proper C hex notation

For cartesian bots, the X_AXIS is the real X movement and same for
Y_AXIS.
But for corexy bots, that is not true. The "X_AXIS" and "Y_AXIS" motors
(that should be named to A_AXIS
and B_AXIS) cannot be used for X and Y length, because A=X+Y and B=X-Y.
So we need to create other 2 "AXIS", named X_HEAD and Y_HEAD, meaning
the real displacement of the Head.
Having the real displacement of the head, we can calculate the total
movement length and apply the desired speed.

Borrow code from https://github.com/arduino/Arduino/issues/1793

Improvement to avoid reinitializing delay buffer with every print. Fixed
issues in buffer indexing and memory out of bounds due to floating point
imprecision.  Simplified the code by avoiding conversion to standard
diameter and 1cu mm extrusion, which caused complications in determining
mm extruded.

This feature allows the printer to read the filament diameter
automatically and adjust the printer in real time.  Added code to read
an analog voltage that represents a filament diameter measurement.  This
measurement is delayed in a ring buffer to compensate for sensors that
are a distance away from the extruder.  The measurement is used to
adjust the volumetric_multiplier for the extruder.  Some additional g
codes (M404, M405, M406, M407) are used to set parameters and turn
on/off the control. g code M221 is updated.  Pins for RAMPS1.4, RAMBO,
and Printrboard are identified for analog input.  The configuration file
is updated with relevant user parameters.

sorry for that

FW retraction is extended onto swap retraction invoked by 'G10 S1'.
Bookkeeping of the retract state of all extruders allows for having one
extruder fw standard retracted while another extruder is swap retracted.
An LCD menu item for the swap retract and recover length was added.

Extension of M221, Tune menu

Save above configs in eeprom
fix docs in createTemperatureLookupMarlin.py
add missing azteegX3pro digipot settings in delta example config

Add digipot i2c control for MCP4451
Allow M907 to set i2c digipot currents in amps
Fix Makefile to allow Azteeg motherboards
Fix Makefile to allow Wire libraries only
Add beeper pin for Azteeg X3 Pro

This is why I wanted to sleep on the code I wrote while falling asleep
rather than immediately submitting a pull request.

Recommended for those who are using the Z Probe for Z Homing (as
Z-Endstop)

This feature has two changes:

1) Allow user to choose where the Z Probe will touch the bed when homing
all axis together (G28) by setting below defines:

Z_SAFE_HOMING_X_POINT
Z_SAFE_HOMING_Y_POINT

2) Prevents the user to perform Z Axis Homing when the Z Probe is
outsite bed.

Added AT90USB device BT connectivity support using second UART.

Included support for BT dongle on AT90USB boards.
Added LCD Shift Register LCD control
Included support for RepRap Clone Wars project board (SAV MKI).

Enables two stepper drivers to be used for the Y axis (useful for
Shapeoko style machines)
Each Y driver can be stepped in either the same way or in opposite
directions, accounting for different hardware setups (leadscrew vs. belt
driven)