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/boards/versapad_bootloader.json

@@ -0,0 +1,20 @@
+{
+  "build": {
+    "core": "N/A",
+    "cpu": "cortex-m0plus",
+    "extra_flags": "-D__SAMD21G17A__",
+    "f_cpu": "48000000L",
+    "mcu": "samd21g17d"
+  },
+  "connectivity": ["usb"],
+  "frameworks": [],
+  "name": "VersaPad v2 (UF2 bootloader, Atmel-ICE build target)",
+  "upload": {
+    "maximum_ram_size": 16384,
+    "maximum_size": 8192,
+    "protocol": "atmel-ice",
+    "require_upload_port": false
+  },
+  "url": "",
+  "vendor": "Custom"
+}