Minor re-factoring. Fix an issue with parking and spindle restore.

- Altered the report counters to be count down, rather than count up.
Simplified some of the logic.

- Fixed an issue with parking restore. The spindle state would disable
then reenable.

- Clarified some of the config.h descriptions.

- Moved the compile-time checks from config.h to grbl.h. They don’t
belong in the config.h file.

- Refactored the initialization of the system variables in main.c.
System position and probe position were undefined when power cycled,
but were zero anyway. Added clear vector code to make it explicit.

grbl/config.h

@@ -281,11 +281,11 @@
 // and agressive streaming. There is also a busy and an idle refresh count, which sets up Grbl to send
 // refreshes more often when its not doing anything important. With a good GUI, this data doesn't need
 // to be refreshed very often, on the order of a several seconds.
-// NOTE: The refresh count cannot be set to zero and must be one or greater.
+// NOTE: WCO refresh must be 2 or greater. OVR refresh must be 1 or greater.
 #define REPORT_OVR_REFRESH_BUSY_COUNT 20  // (1-255)
 #define REPORT_OVR_REFRESH_IDLE_COUNT 10  // (1-255) Must be less than or equal to the busy count
-#define REPORT_WCO_REFRESH_BUSY_COUNT 30  // (1-255)
-#define REPORT_WCO_REFRESH_IDLE_COUNT 10  // (1-255) Must be less than or equal to the busy count
+#define REPORT_WCO_REFRESH_BUSY_COUNT 30  // (2-255)
+#define REPORT_WCO_REFRESH_IDLE_COUNT 10  // (2-255) Must be less than or equal to the busy count
 
 // ----- COMPATIBILITY OPTIONS: ------
 // The following options enabled the old-style v0.9 Grbl interface.
@@ -357,11 +357,13 @@
 // Used by variable spindle output only. This forces the PWM output to a minimum duty cycle when enabled.
 // The PWM pin will still read 0V when the spindle is disabled. Most users will not need this option, but
 // it may be useful in certain scenarios. This minimum PWM settings coincides with the spindle rpm minimum
-// setting, like rpm max to max PWM. So the variable spindle pin will not output the voltage range between
-// 0V for disabled and the voltage set by the minimum PWM for minimum rpm.
+// setting, like rpm max to max PWM. This is handy if you need a larger voltage difference between 0V disabled
+// and the voltage set by the minimum PWM for minimum rpm. This difference is 0.02V per PWM value. So, when
+// minimum PWM is at 1, only 0.02 volts separate enabled and disabled. At PWM 5, this would be 0.1V. Keep
+// in mind that you will begin to lose PWM resolution with increased minimum PWM values, since you have less
+// and less range over the total 256 PWM levels to signal different spindle speeds.
 // NOTE: Compute duty cycle at the minimum PWM by this equation: (% duty cycle)=(SPINDLE_MINIMUM_PWM/256)*100
-// Value must be greater than zero.
-// #define SPINDLE_MINIMUM_PWM 5 // Default disabled. Uncomment to enable. Integer (1-255)
+// #define SPINDLE_MINIMUM_PWM 5 // Default disabled. Uncomment to enable. Must be greater than zero. Integer (1-255).
 
 // By default on a 328p(Uno), Grbl combines the variable spindle PWM and the enable into one pin to help
 // preserve I/O pins. For certain setups, these may need to be separate pins. This configure option uses
@@ -511,6 +513,8 @@
 // Enable the '$I=(string)' build info write command. If disabled, any existing build info data must
 // be placed into EEPROM via external means with a valid checksum value. This macro option is useful
 // to prevent this data from being over-written by a user, when used to store OEM product data.
+// NOTE: If disabled and to ensure Grbl can never alter the build info line, you'll also need to enable
+// the SETTING_RESTORE_ALL macro above and remove SETTINGS_RESTORE_BUILD_INFO from the mask.
 // NOTE: See the included grblWrite_BuildInfo.ino example file to write this string seperately.
 #define ENABLE_BUILD_INFO_WRITE_COMMAND // '$I=' Default enabled. Comment to disable.
 
@@ -558,34 +562,7 @@
 #define PARKING_PULLOUT_RATE 100.0 // Pull-out/plunge slow feed rate in mm/min.
 #define PARKING_PULLOUT_INCREMENT 5.0 // Spindle pull-out and plunge distance in mm. Incremental distance.
                                       // Must be positive value or equal to zero.
-
-
-// ---------------------------------------------------------------------------------------
-// COMPILE-TIME ERROR CHECKING OF DEFINE VALUES:
-
-#ifndef HOMING_CYCLE_0
-  #error "Required HOMING_CYCLE_0 not defined."
-#endif
-
-#if defined(USE_SPINDLE_DIR_AS_ENABLE_PIN) && !defined(VARIABLE_SPINDLE)
-  #error "USE_SPINDLE_DIR_AS_ENABLE_PIN may only be used with VARIABLE_SPINDLE enabled"
-#endif
-
-#if defined(USE_SPINDLE_DIR_AS_ENABLE_PIN) && !defined(CPU_MAP_ATMEGA328P)
-  #error "USE_SPINDLE_DIR_AS_ENABLE_PIN may only be used with a 328p processor"
-#endif
-
-#if defined(PARKING_ENABLE)
-  #if defined(HOMING_FORCE_SET_ORIGIN)
-    #error "HOMING_FORCE_SET_ORIGIN is not supported with PARKING_ENABLE at this time."
-  #endif
-#endif
-
-#if defined(SPINDLE_MINIMUM_PWM)
-  #if !(SPINDLE_MINIMUM_PWM > 0)
-    #error "SPINDLE_MINIMUM_PWM must be greater than zero."
-  #endif
-#endif
+                                      
 
 /* ---------------------------------------------------------------------------------------
    OEM Single File Configuration Option