Updated limit/homing routine. Works, but needs more TLC.

- Added acceleration to the homing routine.

- Homing now accounts for different step rates when moving multiple
axes without exceeding acceleration limits.

- Homing now updates all internal positioning variables to machine zero
after completion.

- "Poor-man's" debounce delay added.

- Updated the delay_us() function to perform faster and more accurate
microsecond delays. Previously, the single increments would add
noticeable time drift for larger delays.

- Fix a bug in the stepper.c prescalar calculations that was changed in
the last commit.

- Other minor fixes.

motion_control.c

@@ -23,6 +23,7 @@
 #include <avr/io.h>
 #include "settings.h"
 #include "config.h"
+#include "gcode.h"
 #include "motion_control.h"
 #include <util/delay.h>
 #include <math.h>
@@ -192,9 +193,12 @@ void mc_dwell(double seconds)
 }
 
 
-// TODO: Update limits and homing cycle subprograms for better integration with new features.
+// Execute homing cycle to locate and set machine zero.
 void mc_go_home()
 {
-  limits_go_home();  
-  plan_set_current_position(0,0,0);
+  limits_go_home();
+  // Upon completion, reset all internal position vectors (g-code parser, planner, system)
+  gc_clear_position();
+  plan_clear_position();
+  clear_vector_double(sys.position);
 }