An undocumented hw bug makes the UART lose chars when RX ISR is disabled, even for a very small amount of time. This happens when RX_BUFFER > 256, and the result is corrupted received commands. Solved by implementing pseudo-atomic operations on 16bit indices.

Pointed out by @GMagician

* 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

Fix #9906

* Work around for serial buffer over run

PronterFace sends a lot of M105 commands.  During long operations such
as UBL's G29 P1, G29 P2, G29 P4 and G26 this can over run the serial
buffer.   This results (very often) in a M1 (actually a M1M105) ending
up in the command queue.

Until we figure out a better way to resolve this issue, this will keep
the UBL commands from experiencing bogus commands at thier completion.

Instead of using const char, it is better to use uint8_t

Compiler error, added char c variable

Fix double character sent when TX buffer size is 0 on mega2560

From @ejtagle, originally #7459

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

These '{' are important to avoid the redefinition of `unsigned char _sreg = SREG;`
at the same level.
Used in
```
#define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
```

Similar to the current Arduino HardwareSerial
but with max. 256 byte buffer-size.

Deactivated by default.

The boards with AT90USB processor (USBCON) already use a TX-buffer.