Patched in work of Parking and unparking of https://github.com/buildrob/Marlin_M600/ allowing Parking/Unparking of head An experimental feature to allow you to park the extruder head by safely moving it to a parking position.

You can use this function for instance to clean the drive gear or change filament. Changing filament allows you to replace a nearly depleted spool or to change the filament entirely for multi-colored prints.

*   M600 - Park head to allow filament change or pause of print
*   M601 - Unpark head to resume printing
*   M602 - Turn on/off extruder motors (e.g., for cleaning)
*   M603 - Display LCD alert, sound beeper and wait for button press.

*   Patches LCD enqueuecommand() support for Multiline operation correctly
*   Adds improved pause function to SDCard/LCD Pause function

From buildrobs original commit:
"This change split the M600 Change Filament command into multiple
primitives to allow its use on machines without LCDs or when being
driven from a host. The split also allows greater flexibility in what is done when the extruders are parked.
I refactored the command to reduce its RAM usage by using the existing destination structure instead of defining a new target. I set the default homing E axis rate to be the default rate used in the ULTIPANEL code (20*60)."