Introduce STACK_GUARD_MARGIN in all variants

Create a gap between the BSS and the stack guard.
Set this gap (STACK_GUARD_MARGIN) to 32 bytes in all variants.

The gap serves two purposes:

- Detect a stack overflow earlier (falsely triggering in overtight
  situations is OK!), so that we can hopefully avoid smashing
  the heap and have a clean view during the dump.

- Reserve spack space itself for the stack dumping machinery,
  which is going to grow the stack even further.

Remove get_stack_guard_test_value() which was unused.

Firmware/SdFatUtil.cpp

@@ -48,24 +48,16 @@ void SdFatUtil::set_stack_guard()
 {	
 	uint32_t *stack_guard;
 
-	stack_guard = (uint32_t*)&__bss_end;
+	stack_guard = (uint32_t*)(&__bss_end + STACK_GUARD_MARGIN);
     *stack_guard = STACK_GUARD_TEST_VALUE;
 }
 
 bool SdFatUtil::test_stack_integrity()
 {
-	uint32_t* stack_guard = (uint32_t*)&__bss_end;
+	uint32_t* stack_guard = (uint32_t*)(&__bss_end + STACK_GUARD_MARGIN);
 	return (*stack_guard == STACK_GUARD_TEST_VALUE);
 }
 
-uint32_t SdFatUtil::get_stack_guard_test_value()
-{
-	uint32_t* stack_guard;
-	uint32_t output;
-	stack_guard = (uint32_t*)&__bss_end;
-	output = *stack_guard;
-	return(output);
-}
 //------------------------------------------------------------------------------
 /** %Print a string in flash memory.
  *

Firmware/SdFatUtil.h

@@ -41,7 +41,6 @@ namespace SdFatUtil {
   void SerialPrintln_P(PGM_P str);
   void set_stack_guard();
   bool test_stack_integrity();
-  uint32_t get_stack_guard_test_value();
 }
 
 using namespace SdFatUtil;  // NOLINT

Firmware/variants/1_75mm_MK2-RAMBo10a-E3Dv6full.h

@@ -418,6 +418,7 @@ THERMISTORS SETTINGS
 #endif
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK2-RAMBo13a-E3Dv6full.h

@@ -407,6 +407,7 @@ THERMISTORS SETTINGS
 #endif
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK25-RAMBo10a-E3Dv6full.h

@@ -469,6 +469,7 @@
 #define TEMP_SENSOR_PINDA 1
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK25-RAMBo13a-E3Dv6full.h

@@ -470,6 +470,7 @@
 #define TEMP_SENSOR_PINDA 1
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK25S-RAMBo10a-E3Dv6full.h

@@ -469,6 +469,7 @@
 #define TEMP_SENSOR_PINDA 1
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK25S-RAMBo13a-E3Dv6full.h

@@ -470,6 +470,7 @@
 #define TEMP_SENSOR_PINDA 1
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK3-EINSy10a-E3Dv6full.h

@@ -598,6 +598,7 @@
 #define TEMP_SENSOR_AMBIENT 2000
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50

Firmware/variants/1_75mm_MK3S-EINSy10a-E3Dv6full.h

@@ -602,6 +602,7 @@
 #define TEMP_SENSOR_AMBIENT 2000
 
 #define STACK_GUARD_TEST_VALUE 0xA2A2
+#define STACK_GUARD_MARGIN     32
 
 #define MAX_BED_TEMP_CALIBRATION 50
 #define MAX_HOTEND_TEMP_CALIBRATION 50