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.

nuts_bolts.c

@@ -46,8 +46,23 @@ void delay_ms(uint16_t ms)
 }
 
 // Delays variable defined microseconds. Compiler compatibility fix for _delay_us(),
-// which only accepts constants in future compiler releases.
+// which only accepts constants in future compiler releases. Written to perform more 
+// efficiently with larger delays, as the counter adds parasitic time in each iteration.
 void delay_us(uint16_t us) 
 {
-  while ( us-- ) { _delay_us(1); }
+  while (us) {
+    if (us < 10) { 
+      _delay_us(1);
+      us--;
+    } else if (us < 100) {
+      _delay_us(10);
+      us -= 10;
+    } else if (us < 1000) {
+      _delay_us(100);
+      us -= 100;
+    } else {
+      _delay_ms(1);
+      us -= 1000;
+    }
+  }
 }