Add UF2 bootloader for USB firmware flashing

Vendored and trimmed from microsoft/uf2-samdx1, adapted to the actual
ATSAMD21G17D (128 KiB flash / 16 KiB RAM), fixing the flash/RAM-size
mismatch that the previously commented-out SAM-BA bootloader target
had (it assumed a 256 KiB SAMD21G18A). Builds as a standalone
PlatformIO environment (bootloader/platformio.ini), no python2/make
dependency. Compiles clean, fits in the 8 KiB bootloader region
(7292/8192 bytes). Not yet flashed/verified on real hardware.

bootloader/include/board_config.h

@@ -0,0 +1,32 @@
+/*
+ * Board config for the VersaPad v2 UF2 bootloader.
+ * Adapted from Microsoft uf2-samdx1 boards/gemma_m0/board_config.h
+ * (closest upstream template: crystalless ATSAMD21, same USB_VID).
+ */
+#ifndef BOARD_CONFIG_H
+#define BOARD_CONFIG_H
+
+#define CRYSTALLESS    1
+
+#define VENDOR_NAME "Custom"
+#define PRODUCT_NAME "VersaPad v2"
+#define VOLUME_LABEL "VERSABOOT"
+#define INDEX_URL "https://git.jappel.io/jappel/VersaMCU"
+#define BOARD_ID "SAMD21G17D-VersaPad-v2"
+
+/* Same VID as the app firmware (platformio.ini), distinct PID so the
+ * bootloader enumerates as a different USB device than the app. */
+#define USB_VID 0x239A
+#define USB_PID 0x0011
+
+/* No plain GPIO status LED on this board, only a WS2812 chain on PB22.
+ * The bootloader's LED_PIN code just toggles a digital output, that would
+ * not drive WS2812 correctly, so leave it undefined for now. */
+//#define LED_PIN
+//#define LED_TX_PIN
+//#define LED_RX_PIN
+
+//#define BOARD_RGBLED_CLOCK_PIN
+//#define BOARD_RGBLED_DATA_PIN
+
+#endif

bootloader/include/uf2_version.h

@@ -0,0 +1 @@
+#define UF2_VERSION_BASE "versapad-1"