add ```^ !(n_ENABLE_ON)``` for each stepper

  * "G1 X1 ; test" was not executing "G1 X1" due to never leaving comment mode.
  * "M117 Hello \;)" printed "Hello \" to the display due to not replacing serial_char properly.

Tested with the following commands:
    * M117 Hello ; test => displays "Hello" on display, ignores "test"
    * G1 X1 ; foo => moves 1mm in X, ignores "foo"
    * ; test => completely ignored, not even acknowledged
    * M117 Hello \;) => displays "Hello ;)" on display
    * M117 Hello \\;) => displays "Hello \" on display, ignores ")"

Autoretract was broken, looks like it was overlooked when the retracted flag was updated for multi extruders.

- Setting boundaries based on configured limits, contrasting with G28
(which forgets these limits?)

- Adapted “Enhanced G29” code referred to in #1499 and posted at
[3dprintboard.com](http://3dprintboard.com/showthread.php?3105-Auto_Bed_
Leveling-Enhanced-G29-command)
- Compatible with current G29 while adding some new arguments
  - `V` sets the verbose level for serial out
  - `T` (or `V` > 2) send a Topology report to serial out
  - `E` works the same way as before
  - `P` works as before (source used `n` or `U`/`u`)
  - `L`, `R`, `B`, `F` work as before
- Still needs sanity checking for `LRBF`

The X_Axis could not home to min while Y_Max endstop was trigged.

- Apply standards to cardreader.*
- Fix minor issues with cardreader.cpp
- Apply standards to some other stale regions

The encrot values are used in only one place, and don’t vary, so remove
redundant references.

- Do field grouping
- Tweak configurations for better grouping

- Parse #if, #ifdef, etc. for rules to enable fields

- Both TMC, L6470 and pin-driven drivers work.
- All can be mixed and matched as needed.

- Reduce calls to millis()
- General cleanup of manage_heaters
- General cleanup of pid autotune
- Formatting here & there
- Macros to clean up and shrink ISR code (reduced by ~364 lines)

Added a second sensor and made them configurable.

Am I the only one who always need to change the thermistortable.h to add
a dummy table??
Ok, it is done. Just set the thermistor to 999 and it will always read
25C. Great for Testing or Development purposes.