See #11097

* Fix G29 (ABL) retry behavior

To allow for the case where a Z endstop (e.g., hall effect) is higher than the probe trigger point, don't check the Z min endstop during probe moves. See #11134

- Allow planner to alter the deceleration phase of the currently executing block.
- Remove BUSY flag, as it is NON ATOMIC to set bits in the Stepper ISR and Planner at the same time.

Co-Authored-By: J.C. Nelson <xc0000005@users.noreply.github.com>

Fixed for cycle used to initialize default values

...and recommended pulse width configuration for A4988

Co-Authored-By: p3p <p3p@p3psoft.co.uk>

* Planner: Removal of some race conditions between Stepper ISR and Planner, some of them pointed out by @AnHardt, some of my own findings
* Fixing timing on stepper pulses adding a compensation for the non-null time required to set ports.

See https://github.com/MarlinFirmware/Marlin/pull/11079#issuecomment-399309709

-Enable boards_dir option in PIO INI
-Remove defunct custom board file operations for PIO auto-build environment
-Remove now-defunct Teensy++ 2.0 PIO entry, replaced by `at90USB1286_DFU` and `at90USB1286_DFU` (using this entry always put AT90USB board boot loaders in jeopardy of being overwritten due to the very small size of the official Teensy++ 2.0 bootloader)
-whitespace

According to https://github.com/MarlinFirmware/Marlin/issues/9917#issuecomment-399204568