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

* fix for #8669

* Update G28.cpp

Possibly more should be brought over? Discussion at #8471

Addressing https://github.com/MarlinFirmware/Marlin/pull/8575#discussion_r154536617

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) add explanations as to why software SPIs are selected

2) remove unused flag/option

still trying to fix file name

This reverts commit 824980e70e, reversing
changes made to aa7efb96bf.

Address #8573, #8595

-Report SERIAL_XON_XOFF in Extended Capabilites M115 report for hosts
-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines
-Update error message conditions

More corrections to make that happen...

G26 uses logic that looks like:
```
ex = _GET_MESH_X(i + 1) - (SIZE_OF_INTERSECTION_CIRCLES - (SIZE_OF_CROSSHAIRS));
```
The Bi-Linear macros need parenthesis to work correctly.