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.

motion_control.c

@@ -21,15 +21,15 @@
 */
 
 #include <avr/io.h>
+#include <util/delay.h>
+#include <math.h>
+#include <stdlib.h>
 #include "settings.h"
 #include "config.h"
 #include "gcode.h"
 #include "motion_control.h"
 #include "spindle_control.h"
 #include "coolant_control.h"
-#include <util/delay.h>
-#include <math.h>
-#include <stdlib.h>
 #include "nuts_bolts.h"
 #include "stepper.h"
 #include "planner.h"
@@ -61,18 +61,28 @@ void mc_line(float x, float y, float z, float feed_rate, uint8_t invert_feed_rat
   do {
     protocol_execute_runtime(); // Check for any run-time commands
     if (sys.abort) { return; } // Bail, if system abort.
-  } while ( plan_check_full_buffer() );  
-  plan_buffer_line(x, y, z, feed_rate, invert_feed_rate);
-  
-  // Auto-cycle start immediately after planner finishes. Enabled/disabled by grbl settings. During 
-  // a feed hold, auto-start is disabled momentarily until the cycle is resumed by the cycle-start 
-  // runtime command.
-  // NOTE: This is allows the user to decide to exclusively use the cycle start runtime command to
-  // begin motion or let grbl auto-start it for them. This is useful when: manually cycle-starting
-  // when the buffer is completely full and primed; auto-starting, if there was only one g-code 
-  // command sent during manual operation; or if a system is prone to buffer starvation, auto-start
-  // helps make sure it minimizes any dwelling/motion hiccups and keeps the cycle going. 
-  if (sys.auto_start) { st_cycle_start(); }
+  } while ( plan_check_full_buffer() );
+
+  // If in check gcode mode, prevent motion by blocking planner.
+  if (bit_isfalse(gc.switches,BITFLAG_CHECK_GCODE)) {
+    plan_buffer_line(x, y, z, feed_rate, invert_feed_rate);
+    
+    // Indicate to the system there is now a planned block in the buffer ready to cycle start.
+    // NOTE: If homing cycles are enabled, a position lost state will lock out all motions,
+    // until a homing cycle has been completed. This is a safety feature to help prevent 
+    // the machine from crashing.
+    if (!sys.state) { sys.state = STATE_QUEUED; }
+    
+    // Auto-cycle start immediately after planner finishes. Enabled/disabled by grbl settings. During 
+    // a feed hold, auto-start is disabled momentarily until the cycle is resumed by the cycle-start 
+    // runtime command.
+    // NOTE: This is allows the user to decide to exclusively use the cycle start runtime command to
+    // begin motion or let grbl auto-start it for them. This is useful when: manually cycle-starting
+    // when the buffer is completely full and primed; auto-starting, if there was only one g-code 
+    // command sent during manual operation; or if a system is prone to buffer starvation, auto-start
+    // helps make sure it minimizes any dwelling/motion hiccups and keeps the cycle going. 
+    if (sys.auto_start) { st_cycle_start(); }
+  }
 }
 
 
@@ -195,14 +205,19 @@ void mc_dwell(float seconds)
 }
 
 
-// Execute homing cycle to locate and set machine zero.
+// Perform homing cycle to locate and set machine zero. Only '$H' executes this command.
+// NOTE: There should be no motions in the buffer and Grbl must be in an idle state before
+// executing the homing cycle. This prevents incorrect buffered plans after homing.
 void mc_go_home()
 {
-  plan_synchronize();  // Empty all motions in buffer before homing.
+  sys.state = STATE_HOMING; // Set system state variable
   PCICR &= ~(1 << LIMIT_INT);   // Disable hard limits pin change interrupt
-  plan_clear_position();
-
+  
   limits_go_home(); // Perform homing routine.
+  if (sys.abort) {
+    sys.state = STATE_LOST; // Homing routine did not complete.
+    return; 
+  }
 
   // The machine should now be homed and machine zero has been located. Upon completion, 
   // reset planner and system internal position vectors, but not gcode parser position yet.
@@ -227,6 +242,7 @@ void mc_go_home()
 
   // If hard limits feature enabled, re-enable hard limits interrupt after homing cycle.
   if (bit_istrue(settings.flags,BITFLAG_HARD_LIMIT_ENABLE)) { PCICR |= (1 << LIMIT_INT); }
+  sys.state = STATE_IDLE; // Finished! 
 }