New startup script setting. New dry run, check gcode switches. New system state variable. Lots of reorganizing.

(All v0.8 features installed. Still likely buggy, but now thourough
testing will need to start to squash them all. As soon as we're done,
this will be pushed to master and v0.9 development will be started.
Please report ANY issues to us so we can get this rolled out ASAP.)

- User startup script! A user can now save one (up to 5 as compile-time
option) block of g-code in EEPROM memory. This will be run everytime
Grbl resets. Mainly to be used as a way to set your preferences, like
G21, G54, etc.

- New dry run and check g-code switches. Dry run moves ALL motions at
rapids rate ignoring spindle, coolant, and dwell commands. For rapid
physical proofing of your code. The check g-code switch ignores all
motion and provides the user a way to check if there are any errors in
their program that Grbl may not like.

- Program restart! (sort of). Program restart is typically an advanced
feature that allows users to restart a program mid-stream. The check
g-code switch can perform this feature by enabling the switch at the
start of the program, and disabling it at the desired point with some
minimal changes.

- New system state variable. This state variable tracks all of the
different state processes that Grbl performs, i.e. cycle start, feed
hold, homing, etc. This is mainly for making managing of these task
easier and more clear.

- Position lost state variable. Only when homing is enabled, Grbl will
refuse to move until homing is completed and position is known. This is
mainly for safety. Otherwise, it will let users fend for themselves.

- Moved the default settings defines into config.h. The plan is to
eventually create a set of config.h's for particular as-built machines
to help users from doing it themselves.

- Moved around misc defines into .h files. And lots of other little
things.

stepper.c

@@ -22,19 +22,12 @@
 /* The timer calculations of this module informed by the 'RepRap cartesian firmware' by Zack Smith
    and Philipp Tiefenbacher. */
 
+#include <avr/interrupt.h>
 #include "stepper.h"
 #include "config.h"
 #include "settings.h"
-#include <math.h>
-#include <stdlib.h>
-#include <util/delay.h>
-#include "nuts_bolts.h"
-#include <avr/interrupt.h>
 #include "planner.h"
 
-// Some useful constants
-#define TICKS_PER_MICROSECOND (F_CPU/1000000)
-#define CYCLES_PER_ACCELERATION_TICK ((TICKS_PER_MICROSECOND*1000000)/ACCELERATION_TICKS_PER_SECOND)
 
 // Stepper state variable. Contains running data and trapezoid variables.
 typedef struct {
@@ -94,7 +87,7 @@ void st_wake_up()
   } else { 
     STEPPERS_DISABLE_PORT &= ~(1<<STEPPERS_DISABLE_BIT);
   }
-  if (sys.cycle_start) {
+  if (sys.state == STATE_CYCLE) {
     // Initialize stepper output bits
     out_bits = (0) ^ (settings.invert_mask); 
     // Initialize step pulse timing from settings. Here to ensure updating after re-writing.
@@ -176,7 +169,7 @@ ISR(TIMER1_COMPA_vect)
     // Anything in the buffer? If so, initialize next motion.
     current_block = plan_get_current_block();
     if (current_block != NULL) {
-      if (!sys.feed_hold) { 
+      if (sys.state == STATE_CYCLE) {
         // During feed hold, do not update rate and trap counter. Keep decelerating.
         st.trapezoid_adjusted_rate = current_block->initial_rate;
         set_step_events_per_minute(st.trapezoid_adjusted_rate); // Initialize cycles_per_step_event
@@ -190,7 +183,6 @@ ISR(TIMER1_COMPA_vect)
       st.step_events_completed = 0;     
     } else {
       st_go_idle();
-      sys.cycle_start = false;
       bit_true(sys.execute,EXEC_CYCLE_STOP); // Flag main program for cycle end
     }    
   } 
@@ -224,7 +216,7 @@ ISR(TIMER1_COMPA_vect)
 
     // While in block steps, check for de/ac-celeration events and execute them accordingly.
     if (st.step_events_completed < current_block->step_event_count) {
-      if (sys.feed_hold) {
+      if (sys.state == STATE_HOLD) {
         // Check for and execute feed hold by enforcing a steady deceleration from the moment of 
         // execution. The rate of deceleration is limited by rate_delta and will never decelerate
         // faster or slower than in normal operation. If the distance required for the feed hold 
@@ -240,7 +232,6 @@ ISR(TIMER1_COMPA_vect)
             // remain intact to ensure the stepper path is exactly the same. Feed hold is still
             // active and is released after the buffer has been reinitialized.
             st_go_idle();
-            sys.cycle_start = false;
             bit_true(sys.execute,EXEC_CYCLE_STOP); // Flag main program that feed hold is complete.
           } else {
             st.trapezoid_adjusted_rate -= current_block->rate_delta;
@@ -430,24 +421,18 @@ static void set_step_events_per_minute(uint32_t steps_per_minute)
 // variable will manage all of Grbl's processes and keep them separate.
 void st_cycle_start() 
 {
-  if (!sys.cycle_start) {
-    if (!sys.feed_hold) {
-      if (bit_isfalse(sys.execute,EXEC_ALARM)) {
-        sys.cycle_start = true; // Only place this variable is set true.
-        st_wake_up();
-      }
-    }
+  if (sys.state == STATE_QUEUED) {
+    sys.state = STATE_CYCLE;
+    st_wake_up();
   }
 }
 
 // Execute a feed hold with deceleration, only during cycle. Called by main program.
 void st_feed_hold() 
 {
-  if (!sys.feed_hold) {
-    if (sys.cycle_start) {
-      sys.auto_start = false; // Disable planner auto start upon feed hold.
-      sys.feed_hold = true;
-    }
+  if (sys.state == STATE_CYCLE) {
+    sys.state = STATE_HOLD;
+    sys.auto_start = false; // Disable planner auto start upon feed hold.
   }
 }
 
@@ -466,6 +451,8 @@ void st_cycle_reinitialize()
     set_step_events_per_minute(st.trapezoid_adjusted_rate);
     st.trapezoid_tick_cycle_counter = CYCLES_PER_ACCELERATION_TICK/2; // Start halfway for midpoint rule.
     st.step_events_completed = 0;
+    sys.state = STATE_QUEUED;
+  } else {
+    sys.state = STATE_IDLE;
   }
-  sys.feed_hold = false; // Release feed hold. Cycle is ready to re-start.
 }