Add maintenance mode and board status indicators

Provide BOOT-triggered USB CDC maintenance mode for file updates, plus
onboard LED activity flashes and enhanced display status information.

maintenance.py

@@ -0,0 +1,39 @@
+"""Local USB-REPL maintenance-mode selection for the T-Dongle-S3."""
+
+import time
+from machine import Pin
+
+# The T-Dongle-S3 BOOT switch connects GPIO0 to ground.
+_BUTTON_PIN = 0
+_WINDOW_MS = 1500
+
+active = False
+_button = None
+
+
+def button_pressed():
+    """Return whether the BOOT button is currently pressed."""
+    global _button
+    if _button is None:
+        _button = Pin(_BUTTON_PIN, Pin.IN, Pin.PULL_UP)
+    return not _button.value()
+
+
+def check_button():
+    """Return true when BOOT is pressed shortly after a normal reset.
+
+    Do not hold BOOT while resetting: GPIO0 is also an ESP ROM boot strap and
+    that combination intentionally enters the download bootloader instead.
+    """
+    global active
+    deadline = time.ticks_add(time.ticks_ms(), _WINDOW_MS)
+    while time.ticks_diff(deadline, time.ticks_ms()) > 0:
+        if button_pressed():
+            # Debounce the switch and leave the user enough time to release it.
+            time.sleep_ms(40)
+            if button_pressed():
+                active = True
+                return True
+        time.sleep_ms(20)
+    active = False
+    return False