Always read key to check for new LUFA bootloader

Instead of checking for the NEW_LUFA_SIGNATURE once in program memory and then
setting a flag which is used for further checks, a function is used that always
checks program memory directly.

If a flag is used, there's a slight chance that its location in RAM could fall
on MAGIC_KEY_POS. In this case, an aborted USB auto-reset sequence may fail.

cores/arduino/CDC.cpp

@@ -36,8 +36,6 @@ static volatile int32_t breakValue = -1;
 
 static u8 wdtcsr_save;
 
-bool _updatedLUFAbootloader = false;
-
 #define WEAK __attribute__ ((weak))
 
 extern const CDCDescriptor _cdcInterface PROGMEM;
@@ -59,6 +57,11 @@ const CDCDescriptor _cdcInterface =
 	D_ENDPOINT(USB_ENDPOINT_IN (CDC_ENDPOINT_IN ),USB_ENDPOINT_TYPE_BULK,USB_EP_SIZE,0)
 };
 
+bool isLUFAbootloader()
+{
+	return pgm_read_word(FLASHEND - 1) == NEW_LUFA_SIGNATURE;
+}
+
 int CDC_GetInterface(u8* interfaceNum)
 {
 	interfaceNum[0] += 2;	// uses 2
@@ -108,7 +111,7 @@ bool CDC_Setup(USBSetup& setup)
 #if MAGIC_KEY_POS != (RAMEND-1)
 			// For future boards save the key in the inproblematic RAMEND
 			// Which is reserved for the main() return value (which will never return)
-			if (_updatedLUFAbootloader) {
+			if (isLUFAbootloader()) {
 				// horray, we got a new bootloader!
 				magic_key_pos = (RAMEND-1);
 			}

cores/arduino/USBCore.cpp

@@ -35,7 +35,6 @@ extern const u16 STRING_LANGUAGE[] PROGMEM;
 extern const u8 STRING_PRODUCT[] PROGMEM;
 extern const u8 STRING_MANUFACTURER[] PROGMEM;
 extern const DeviceDescriptor USB_DeviceDescriptorIAD PROGMEM;
-extern bool _updatedLUFAbootloader;
 
 const u16 STRING_LANGUAGE[2] = {
 	(3<<8) | (2+2),
@@ -819,12 +818,6 @@ void USBDevice_::attach()
 	UDIEN = (1<<EORSTE) | (1<<SOFE) | (1<<SUSPE);	// Enable interrupts for EOR (End of Reset), SOF (start of frame) and SUSPEND
 	
 	TX_RX_LED_INIT;
-
-#if MAGIC_KEY_POS != (RAMEND-1)
-	if (pgm_read_word(FLASHEND - 1) == NEW_LUFA_SIGNATURE) {
-		_updatedLUFAbootloader = true;
-	}
-#endif
 }
 
 void USBDevice_::detach()

cores/arduino/USBCore.h

@@ -285,8 +285,7 @@ typedef struct
 // Old Caterina bootloader places the MAGIC key into unsafe RAM locations (it can be rewritten
 // by the running sketch before to actual reboot).
 // Newer bootloaders, recognizable by the LUFA "signature" at the end of the flash, can handle both
-// the usafe and the safe location. Check once (in USBCore.cpp) if the bootloader in new, then set the global
-// _updatedLUFAbootloader variable to true/false and place the magic key consequently
+// the usafe and the safe location.
 #ifndef MAGIC_KEY
 #define MAGIC_KEY 0x7777
 #endif