Z babystepping for coreXY (PR#47)

only X and Y were not implemented

Marlin/Configuration_adv.h

@@ -362,6 +362,7 @@
 //#define BABYSTEPPING
 #if ENABLED(BABYSTEPPING)
   #define BABYSTEP_XY  //not only z, but also XY in the menu. more clutter, more functions
+                       //not implemented for CoreXY and deltabots!
   #define BABYSTEP_INVERT_Z false  //true for inverse movements in Z
   #define BABYSTEP_Z_MULTIPLICATOR 2 //faster z movements
 #endif

Marlin/SanityCheck.h

@@ -32,8 +32,8 @@
    * Babystepping
    */
   #if ENABLED(BABYSTEPPING)
-    #if ENABLED(COREXY)
-      #error BABYSTEPPING not implemented for COREXY yet.
+    #if ENABLED(COREXY) && ENABLED(BABYSTEP_XY)
+      #error BABYSTEPPING only implemented for Z axis on CoreXY.
     #endif
     #if ENABLED(SCARA)
       #error BABYSTEPPING is not implemented for SCARA yet.