add watch_reset_to_bootloader function (resolves #81)

watch-library/hardware/watch/watch.c

@@ -45,3 +45,9 @@ bool watch_is_buzzer_or_led_enabled(void){
 bool watch_is_usb_enabled(void) {
     return USB->DEVICE.CTRLA.bit.ENABLE;
 }
+
+void watch_reset_to_bootloader(void) {
+    volatile uint32_t *dbl_tap_ptr = ((volatile uint32_t *)(HSRAM_ADDR + HSRAM_SIZE - 4));
+    *dbl_tap_ptr = 0xf01669ef; // from the UF2 bootloaer: uf2.h line 255
+    NVIC_SystemReset();
+}

watch-library/shared/watch/watch.h

@@ -80,6 +80,10 @@ bool watch_is_buzzer_or_led_enabled(void);
   */
 bool watch_is_usb_enabled(void);
 
+/** @brief Resets in the UF2 bootloader mode
+  */
+void watch_reset_to_bootloader(void);
+
 /** @brief Reads up to len bytes from the USB serial.
   * @param file ignored, you can pass in 0
   * @param ptr pointer to a buffer of at least len bytes

watch-library/simulator/watch/watch.c

@@ -7,3 +7,7 @@ bool watch_is_buzzer_or_led_enabled(void) {
 bool watch_is_usb_enabled(void) {
     return true;
 }
+
+void watch_reset_to_bootloader(void) {
+    // No bootloader in the simulator; nothing to do here
+}