- Stepper bugs fixed
- Support MIXING_EXTRUDER with Linear Advance
- Miscellaneous cleanup

* Misc fixes and improvements

- Get rid of most critical sections on the Serial port drivers for AVR and DUE. Proper usage of FIFOs should allow interrupts to stay enabled without harm to queuing and dequeuing.
  Also, with 8-bit indices (for AVR) and up to 32-bit indices (for ARM), there is no need to protect reads and writes to those indices.
- Simplify the XON/XOFF logic quite a bit. Much cleaner now (both for AVR and ARM)
- Prevent a race condition (edge case) that could happen when estimating the proper value for the stepper timer (by reading it) and writing the calculated value for the time to the next ISR by disabling interrupts in those critical and small sections of the code - The problem could lead to lost steps.
- Fix dual endstops not properly homing bug (maybe).

* Set position immediately when possible

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

- Also implemented real endstop reading on interrupt.

Better encapsulation and considerably reduce stepper jitter

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

Enable 6th-order jerk-controlled motion planning in real-time.
Only for 32bit MCUs. (AVR simply does not have enough processing power for this!)

To reduce confusion over the current timer count vs. the compare (aka "top") value. Caution: this re-uses the function name, changing its meaning.

This reverts commit 5cf6a062e3.

Fix #8710

* Merging early because of build failures.  See #8105

* Make LPC1768 pinmapping not specific to Re-ARM

* Add HAL_PIN_TYPE and LPC1768 pin features

* M43 Updates

* Move pin map into pinsDebug_LPC1768.h

* Incorporate comments and M226

* Fix persistent store compilation issues

* Update pin features

* Update MKS SBASE pins

* Use native LPC1768 pin numbers in M42, M43, and M226

Alternative to #7882. If F_CPU is greater than 1000 it can be evenly divided by 8. Over 10000, 16; over 100000, 32; over 1 million, 64; etc.

Replace ARDUINO_ARCH_AVR with __AVR__ to better detect architecture for non-Arduino dev environments.  Resolves compile failure in PIO for 8-bit Teensduino targets

More info:
https://forum.pjrc.com/threads/33234-Using-Teensyduino-Selecting-Teensy-3-2-3-1-board-has-incorrect-platform-define
http://www.atmel.com/webdoc/avrlibcreferencemanual/using_tools_1using_avr_gcc_mach_opt.html

Implement AVR Platform