Don't run user application in the bootloader unless a valid app is present (thanks to Alex Kazik).

Bootloaders/Printer/BootloaderPrinter.c

@@ -119,9 +119,49 @@ uint16_t MagicBootKey ATTR_NO_INIT;
  */
 void Application_Jump_Check(void)
 {
+	bool JumpToApplication = false;
+
+	#if (BOARD == BOARD_LEONARDO)
+		/* Enable pull-up on the IO13 pin so we can use it to select the mode */
+		PORTC |=  (1 << 7);
+		Delay_MS(10);
+
+		/* If IO13 is not jumpered to ground, start the user application instead */
+		JumpToApplication |= ((PINC & (1 << 7)) != 0);
+
+		/* Disable pull-up after the check has completed */
+		PORTC &= ~(1 << 7);
+	#elif ((BOARD == BOARD_XPLAIN) || (BOARD == BOARD_XPLAIN_REV1))
+		/* Disable JTAG debugging */
+		JTAG_DISABLE();
+
+		/* Enable pull-up on the JTAG TCK pin so we can use it to select the mode */
+		PORTF |= (1 << 4);
+		Delay_MS(10);
+
+		/* If the TCK pin is not jumpered to ground, start the user application instead */
+		JumpToApplication |= ((PINF & (1 << 4)) != 0);
+
+		/* Re-enable JTAG debugging */
+		JTAG_ENABLE();
+	#endif
+
 	/* If the reset source was the bootloader and the key is correct, clear it and jump to the application */
 	if ((MCUSR & (1 << WDRF)) && (MagicBootKey == MAGIC_BOOT_KEY))
+	  JumpToApplication |= true;
+
+	/* Don't run the user application if the reset vector is blank (no app loaded) */
+	if (pgm_read_word_near(0) == 0xFFFF)
+	  JumpToApplication = false;
+
+	/* If a request has been made to jump to the user application, honor it */
+	if (JumpToApplication)
 	{
+		/* Turn off the watchdog */
+		MCUSR &= ~(1<<WDRF);
+		wdt_disable();
+
+		/* Clear the boot key and jump to the user application */
 		MagicBootKey = 0;
 
 		// cppcheck-suppress constStatement