[1.1.x] Discard "continued" blocks on interrupted move

Based on #8685

Fix - Conditionals_post.h - allow overriding of [min,max] probe point

[1.1.x] MULTIPLE_PROBING — Why stop at 2?

[1.1.x] Purge blocks on endstop/probe hit

[1.1.x] Cleanup, bugfixes, parity with 2.0.x

[1.1.x] Don't split first_move while homing or probing

While homing or probing it might be bad if the stop/trobe triggers during the first part and the second is still in the buffer.

Addressing #8676

[1.1.x] Change K1 to required option PID_K1

Addressing #8662

Added Support for RAMPS 1.4 Plus board

Conflicts:
	Marlin/boards.h

Add missing library in Makefile

When using RELOC_WORKAROUND library needs to be linked explicitly, otherwise
eeprom functions are not available.

Thanks to @rfjakob for finding the crucial hint to solve this issue.

* SERIAL_XON_XOFF not supported on USB-native AVR devices

User could enable SERIAL_XON_XOFF on USB-native devices and it would not be enabled without warning, but M115 would report the capability as available.

[1.1.x] Split first move to planner for better chaining

[1.1.x] Revert PR #8608 — split first planner move

This reverts commit 0eef0ff0de, reversing
changes made to d8582b7506.