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/platformio.ini

@@ -0,0 +1,25 @@
+; VersaPad v2 -- UF2 bootloader
+; Vendored + adapted from Microsoft uf2-samdx1 (https://github.com/microsoft/uf2-samdx1),
+; trimmed to SAMD21 only, no self-update (SELF_EXECUTABLE) build, no Arduino framework.
+; Programmer: Atmel-ICE via SWD, same as the app firmware in ../platformio.ini.
+
+[env:versapad_bootloader]
+platform    = atmelsam
+board       = versapad_bootloader
+build_flags =
+    -D__SAMD21G17A__
+    -DSAMD21
+    -DCRYSTALLESS
+    -DFLASH_NUM_ROWS=512   ; ATSAMD21G17D: 128 KiB / 256 B rows (uf2.h defaults to 1024, the 256 KiB G18A value)
+    -I.
+    -Iinclude
+    -Iinc
+    -Ilib/cmsis/CMSIS/Include
+    -Ilib/samd21/samd21a/include
+    -Ilib/usb_msc
+
+board_build.ldscript = scripts/samd21g17a.ld
+
+upload_protocol = custom
+extra_scripts   = ../upload_openocd.py
+debug_tool      = openocd