New button behaviour in bootloader mode:

- Keep button pressed when powering to force entering bootloader
  - Once in bootloader more, you can release the button
- When in bootloader mode (no matter how you entered that mode)
  - One button press to quit bootloader mode and boot main image

include/proxmark3.h

@@ -128,6 +128,7 @@ struct common_area {
     struct {
         unsigned int bootrom_present: 1; /* Set when a bootrom that is capable of parsing the common area is present */
         unsigned int osimage_present: 1; /* Set when a osimage that is capable of parsing the common area is present */
+        unsigned int button_pressed: 1;
     } __attribute__((packed)) flags;
     int arg1, arg2;
 } __attribute__((packed));