The PrintrBoard Rev F utilizes a mcp4728 DAC to set motor current. Printrbot's implementation utilizes 2 new M-codes to set and write the DAC settings to the DAC EEPROM: `M909` (Read DAC) and `M910` (Write DAC). `M907` is re-used to set value, `M908` for direct control.

The Pins file for the RevF board is included.

Aim: Test probes in update_endstops only when activated

Changes:
Configurations
 Add define for FIX_MOUNTED_PROBE to handle the situation where formerly ENDSTOPS_ONLY_FOR_HOMING had to be set, or lowering the nozzle below Z_PROBE_OFFSET_FROM_EXTRUDER could give an "endstop hit" message.
 Add define for Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN to indicate a common situation, that we have a probe but it is connected to an endstop pin
 Add some comments
 Shift some entries to have related things together.

Conditionals.h
 We have a probe (HAS_Z_MIN_PROBE) if one of the pins is defined AND one of the probes is defined.

SanityCheck.h
 Add some tests if the probe is connected and if we have defined a probe.

stepper.cpp
 Changes to test the probe only when it is deployed (z_probe_is_active).
 Test update_endstops() when the probe is deployed.

MarlinMain.cpp
 a. set and reset z_probe_is_active in deploy_z_probe(), stow_z_probe() and dock_sled()
 b. set and reset z_probe_is_active in the case a z-servo is moved to a defined position. The only remaining unhandled servo move is in M280 where we do not end in a defined position. If you want to handle a probe use M401/402
 c. skip deploying/stowing when already deployed/stowed in the dedicated deploy/stow functions.
 d. Handle the new FIX_MOUNTED_PROBE in parallel to a servo driven probe/endstop.

 To do: In another PR. handle all probes in deploy/stow_z_probe.
 Sort out SERVO_LEVELING vs. HAS_SERVO_ENDSTOPS.

fixes MarlinFirmware/Marlin#3144 and MarlinFirmware/Marlin#3083

Forgot Z2_MIN_PIN

Keep "astyled" reformatting

(PR #138)

When the ISR does not finish in time, the timer will
wrap in the computation of the next interrupt time.

This hack replaces the correct (past) time with a time not far in the future.

* Fix a couple of renames omission and macro expansion errors.

Currently we use the probe exclusively as a device to find the build platform(bed).
For the currently supported setups this means, we use it as a additional min-endstop.
A triggered when not deployed probe disturbs the homing process for max-endstops.

Rename ENDSTOPPULLUP_ZPROBE to ENDSTOPPULLUP_ZMIN_PROBE
Rename Z_PROBE_ENDSTOP_INVERTING to Z_MIN_PROBE_ENDSTOP_INVERTING
Rename Z_PROBE_ENDSTOP to Z_MIN_PROBE_ENDSTOP
Rename DISABLE_Z_PROBE_ENDSTOP to DISABLE_Z_MIN_PROBE_ENDSTOP
Rename Z_PROBE_REPEATABILITY_TEST to Z_MIN_PROBE_REPEATABILITY_TEST
Rename Z_PROBE_ENDSTOP to Z_MIN_PROBE_ENDSTOP
Adjust comments accordingly
Remove Z_MAX  check for the probe in update_endstops().

Using an delta related idea of @clefranc from #61,
extended to the general change for all setups.

Tested with Prusa i3, max-z-endstop and permanently triggered z-probe.
Worked for @clefranc's delta.

Apply `ENABLED` / `DISABLED` macros to stepper-related files.

!(~(ab)&0x3) and ab == 0x3 are equal

to warrant watching is set or reset.

Make setTargetBed() in _lcd_preheat() dependant of TEMP_SENSOR_BED.
Use disable_all_heaters() in lcd_cooldown() and abort_on_endstop_hit.

this PR replaces some bools with one char. this will safe 3 bytes and should also be sometimes a little bit faster.

delete some tabs

I.e., when acceleration * steps per mm > 2,000,000.
This was done by changing MultiU24X24toH16 to take a 32b bit operand.
Removed the claim that stepper.cpp uses the Leib algorithm.