Soft Reset via Serial or post-kill button click (#21652)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/HAL/AVR/HAL.cpp

@@ -58,6 +58,15 @@ void HAL_init() {
   #endif
 }
 
+void HAL_reboot() {
+  #if ENABLED(USE_WATCHDOG)
+    while (1) { /* run out the watchdog */ }
+  #else
+    void (*resetFunc)() = 0;  // Declare resetFunc() at address 0
+    resetFunc();              // Jump to address 0
+  #endif
+}
+
 #if ENABLED(SDSUPPORT)
 
   #include "../../sd/SdFatUtil.h"

Marlin/src/HAL/AVR/HAL.h

@@ -135,7 +135,7 @@ void HAL_init();
 inline void HAL_clear_reset_source() { MCUSR = 0; }
 inline uint8_t HAL_get_reset_source() { return MCUSR; }
 
-inline void HAL_reboot() {}  // reboot the board or restart the bootloader
+void HAL_reboot();
 
 #if GCC_VERSION <= 50000
   #pragma GCC diagnostic push