safe mode

- allows you to reflash the device even if you've uploaded a sketch that
  interferes with the normal reflashing process (corrupts memory, turns
  off power to USB, etc)
- Thanks @Wozza365 for giving me this idea.

This is also solvable by shorting the reset pin but @Wozza365 was having
a heck of a time getting that working because the 8 seconds that you're
given to start the upload post reset were too short.  His computer was
not  recognizing the USB device in that amount of time.  This tiny
change would have allowed him to boot into safe mode and then take his
time following the typical reflash process.

Arduboy.cpp

@@ -76,6 +76,23 @@ void Arduboy::start()
   SPI.transfer(end & 0x07);
 
   LCDDataMode();
+  #ifdef SAFE_MODE
+  if (pressed(LEFT_BUTTON+UP_BUTTON))
+    safeMode();
+  #endif
+}
+
+// Safe Mode is engaged by holding down both the LEFT button and UP button
+// when plugging the device into USB.  It puts your device into a tight
+// loop and allows it to be reprogrammed even if you have uploaded a very
+// broken sketch that interferes with the normal USB triggered auto-reboot
+// functionality of the device.
+void Arduboy::safeMode()
+{
+  display(); // too avoid random gibberish
+  while (true) {
+    asm volatile("nop \n");
+  }
 }
 
 void Arduboy::LCDDataMode()

Arduboy.h

@@ -5,6 +5,7 @@
 #include <Print.h>
 
 #define PIXEL_SAFE_MODE
+#define SAFE_MODE
 
 #define CS 6
 #define DC 4
@@ -69,6 +70,7 @@ class Arduboy : public Print
 
 private:
   unsigned char sBuffer[(HEIGHT*WIDTH)/8];
+  void safeMode();
   uint8_t readCapacitivePin(int pinToMeasure);
   uint8_t readCapXtal(int pinToMeasure);
   volatile uint8_t *mosiport, *clkport, *csport, *dcport;