Protect MarlinSerial against interrupts
by shielding the CRITICAL_SECTIONs

Now with a test if RX_BUFFER_SIZE is in the required range.
Code in peek() and read() is now optimized for readability and showing the similarity between the two.

Maybe a bit overprotected in checkRx() and store_char(), but now some days without detected errors from this source.

Keep "astyled" reformatting

- For some reason it’s easy to forget what BT stands for, but it
doesn’t need an abbreviation.

Apply `ENABLED` / `DISABLED` macros to files needing only a small
number of changes.

- Add BIT and TEST macros
- Add _APPLY_ macros to stepper.cpp to help with consolidation
- Consolidate code in stepper.cpp using macros
- Apply standards in stepper.cpp
- Use >= 0 instead of > -1 as a better semantic
- Replace DUAL_Y_CARRIAGE with Y_DUAL_STEPPER_DRIVERS

Included support for BT dongle on AT90USB boards.
Added LCD Shift Register LCD control
Included support for RepRap Clone Wars project board (SAV MKI).

Assume SERIAL_PORT 0 if not defined. Collapse un-numbered register
special case. Clarify comments.

This change makes the choice of serial port configurable so that
wireless capability can be easily added by connecting Bluetooth modules
(such as BlueSmirf or JY-MCU) to the expansion port pins.

Found a couple of unused variables, that I commented.
Tried to solve the program memory warning message, and failed.

Add serial checkRx in stepper ISR.
Copied HardwareSerial to MarlinSerial (Needed for checkRx).