[2.0.x] Small assorted collection of fixes and improvements (#10911)

* 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

Marlin/src/module/planner.cpp

@@ -2467,9 +2467,13 @@ void Planner::_set_position_mm(const float &a, const float &b, const float &c, c
     position_float[C_AXIS] = c;
     position_float[E_AXIS] = e;
   #endif
-  previous_nominal_speed_sqr = 0.0; // Resets planner junction speeds. Assumes start from rest.
-  ZERO(previous_speed);
-  buffer_sync_block();
+  if (has_blocks_queued()) {
+    //previous_nominal_speed_sqr = 0.0; // Reset planner junction speeds. Assume start from rest.
+    //ZERO(previous_speed);
+    buffer_sync_block();
+  }
+  else
+    stepper.set_position(position[A_AXIS], position[B_AXIS], position[C_AXIS], position[E_AXIS]);
 }
 
 void Planner::set_position_mm_kinematic(const float (&cart)[XYZE]) {
@@ -2501,8 +2505,12 @@ void Planner::set_position_mm(const AxisEnum axis, const float &v) {
   #if HAS_POSITION_FLOAT
     position_float[axis] = v;
   #endif
-  previous_speed[axis] = 0.0;
-  buffer_sync_block();
+  if (has_blocks_queued()) {
+    //previous_speed[axis] = 0.0;
+    buffer_sync_block();
+  }
+  else
+    stepper.set_position(axis, position[axis]);
 }
 
 // Recalculate the steps/s^2 acceleration rates, based on the mm/s^2