[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/HAL/HAL_AVR/MarlinSerial.h

@@ -94,7 +94,7 @@
     extern ring_buffer_pos_t rx_max_enqueued;
   #endif
 
-  class MarlinSerial { //: public Stream
+  class MarlinSerial {
 
     public:
       MarlinSerial() {};
@@ -104,13 +104,10 @@
       static int read(void);
       static void flush(void);
       static ring_buffer_pos_t available(void);
-      static void checkRx(void);
       static void write(const uint8_t c);
       #if TX_BUFFER_SIZE > 0
-        static uint8_t availableForWrite(void);
         static void flushTX(void);
       #endif
-      static void writeNoHandshake(const uint8_t c);
 
       #if ENABLED(SERIAL_STATS_DROPPED_RX)
         FORCE_INLINE static uint32_t dropped() { return rx_dropped_bytes; }