Allow Y to home before X with option in Configuration_adv.h.

This addresses comments in #1956 and #1079. In particular, this is useful
when both endstops are stationary on a CoreXY system, and the Y axis needs
to be homed before the X so the flags are aligned.

Marlin/example_configurations/tvrrug/Round2/Configuration_adv.h

@@ -195,6 +195,9 @@
 #define HOMING_BUMP_DIVISOR {2, 2, 4}  // Re-Bump Speed Divisor (Divides the Homing Feedrate)
 //#define QUICK_HOME  //if this is defined, if both x and y are to be homed, a diagonal move will be performed initially.
 
+// When G28 is called, this option will make Y home before X
+// #define HOME_Y_BEFORE_X
+
 // @section machine
 
 #define AXIS_RELATIVE_MODES {false, false, false, false}