With the introduction of the #if ENABLED(…)

reads better than

* Fix a couple of renames omission and macro expansion errors.

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

The compiler may be able to optimize if it knows an array won’t be
changing.

Note that newer versions of the IDE can still
be made to support compilation for the older boards.

replace extruder_multiply with extruder_multiplier
like feedrate_multiplier or volumetric_multiplier

Makes the output on the LCD adjustable.

- For better encapsulation add inlines for each prepare_move type
- Move controllerFan vars inside the function as statics
- Some formatting cleanup
- Rename `ClearToSend` as `ok_to_send`

this PR replaces some bools with one char. this will safe 3 bytes and should also be sometimes a little bit faster.

- Add `M111` as a standard option to set the debug level
- Implement `DEBUG_ECHO` in `process_commands`
- Other debug levels (e.g., `DEBUG_DRYRUN`) need more work