Add end-to-end USB flashing for the app firmware via UF2

Adds uf2conv.py (minimal, dependency-free .bin -> .uf2 converter
matching bootloader/inc/uf2format.h's block layout) and upload_uf2.py,
a PlatformIO upload hook for env:versapad_usb that finds the mounted
VERSABOOT volume and copies the converted firmware onto it.

env:versapad_usb previously used upload_protocol=sam-ba, the classic
Arduino/Atmel protocol -- the actual bootloader speaks UF2/mass
storage, not SAM-BA, so that upload path never worked. Switched to
upload_protocol=custom with the new hook, and cleaned the now-unused
SAM-BA-specific fields out of boards/versapad.json.

Verified end to end on real hardware: pio run -e versapad_usb
--target upload builds, converts, copies to the VERSABOOT drive, and
the bootloader jumps into the freshly written app on its own.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

boards/versapad.json

@@ -22,13 +22,8 @@
   "upload": {
     "maximum_ram_size": 16384,
     "maximum_size": 120832,
-    "disable_flushing": true,
-    "native_usb": true,
     "offset": "0x2000",
-    "protocol": "sam-ba",
-    "require_upload_port": true,
-    "use_1200bps_touch": true,
-    "wait_for_upload_port": true
+    "protocol": "custom"
   },
   "url": "",
   "vendor": "Custom"