Fix board definition: VersaPad uses SAMD21G17D, not G18A

versapad.json declared samd21g18a (256K flash / 32K RAM) while the
linker script, the NVM config addresses (0x1FB00-0x1FFFF, just below
the 128K boundary) and versapad_nobl.json all point to the actual
chip, a SAMD21G17D (128K flash / 16K RAM). PlatformIO's flash-size
check was silently using a ~248K limit instead of the real ~120K
budget for the USB-bootloader build variant.

boards/versapad.json

@@ -7,12 +7,12 @@
     "core": "arduino",
     "variant": "versapad",
     "cpu": "cortex-m0plus",
-    "extra_flags": "-DARDUINO_SAMD_ZERO -DARM_MATH_CM0PLUS -D__SAMD21G18A__",
+    "extra_flags": "-DARDUINO_SAMD_ZERO -DARM_MATH_CM0PLUS -D__SAMD21G17D__",
     "f_cpu": "48000000L",
     "hwids": [
       ["0x239A", "0x0011"]
     ],
-    "mcu": "samd21g18a",
+    "mcu": "samd21g17d",
     "usb_product": "VersaPad v2",
     "usb_manufacturer": "Custom"
   },
@@ -20,8 +20,8 @@
   "frameworks": ["arduino"],
   "name": "VersaPad v2 (USB bootloader)",
   "upload": {
-    "maximum_ram_size": 32768,
+    "maximum_ram_size": 16384,
-    "maximum_size": 253952,
+    "maximum_size": 122880,
     "disable_flushing": true,
     "native_usb": true,
     "offset": "0x2000",