Move the Z probe as far as needed to trigger the switch (PR#2461)

Marlin/Marlin_main.cpp

       feedrate = homing_feedrate[Z_AXIS];
 
       // Move down until the probe (or endstop?) is triggered
-      float zPosition = -10;
+      float zPosition = -(Z_MAX_LENGTH + 10);
       line_to_z(zPosition);
       st_synchronize();