feat(mcu): Add micropython firmware and build instructions

firmware/OLIMEX_PICO2_XL/board.json

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+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "Dual-core",
+        "External Flash",
+        "USB-C"
+    ],
+    "images": [
+        "rp2-pico2.jpg"
+    ],
+    "mcu": "rp2350",
+    "product": "PICO2-XL",
+    "thumbnail": "",
+    "url": "https://www.olimex.com/Products/RaspberryPi/PICO/PICO2-XXL/open-source-hardware",
+    "variants": {
+        "RISCV": "RISC-V CPU mode"
+    },
+    "vendor": "Olimex"
+}

firmware/OLIMEX_PICO2_XL/mpconfigboard.cmake

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+set(PICO_BOARD "olimex_rp2350_xl")
+set(PICO_NUM_GPIOS 48)

firmware/OLIMEX_PICO2_XL/mpconfigboard.h

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+// Board and hardware specific configuration
+#define MICROPY_HW_BOARD_NAME                   "Olimex PICO2-XL"
+#define MICROPY_HW_FLASH_STORAGE_BYTES (PICO_FLASH_SIZE_BYTES - (1 * 1024 * 1024))
+
+// USB config
+#define MICROPY_HW_USB_MANUFACTURER_STRING "Olimex"
+#define MICROPY_HW_USB_PRODUCT_FS_STRING "PICO2-XL"
+#define MICROPY_HW_USB_MSC (1)
+#define MICROPY_HW_ENABLE_UART_REPL (1)

firmware/OLIMEX_PICO2_XL/mpconfigvariant.cmake

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+set(PICO_PLATFORM "rp2350")

firmware/OLIMEX_PICO2_XL/mpconfigvariant_RISCV.cmake

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+set(PICO_PLATFORM "rp2350-riscv")

firmware/OLIMEX_PICO2_XL/pins.csv

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+GP0,GPIO0
+GP1,GPIO1
+GP2,GPIO2
+GP3,GPIO3
+GP4,GPIO4
+GP5,GPIO5
+GP6,GPIO6
+GP7,GPIO7
+GP8,GPIO8
+GP9,GPIO9
+GP10,GPIO10
+GP11,GPIO11
+GP12,GPIO12
+GP13,GPIO13
+GP14,GPIO14
+GP15,GPIO15
+GP16,GPIO16
+GP17,GPIO17
+GP18,GPIO18
+GP19,GPIO19
+GP20,GPIO20
+GP21,GPIO21
+GP22,GPIO22
+GP25,GPIO25
+GP26,GPIO26
+GP27,GPIO27
+GP28,GPIO28
+GP29,GPIO29
+GP30,GPIO30
+GP31,GPIO31
+GP32,GPIO32
+GP33,GPIO33
+GP34,GPIO34
+GP35,GPIO35
+GP36,GPIO36
+GP37,GPIO37
+GP38,GPIO38
+GP39,GPIO39
+GP40,GPIO40
+GP41,GPIO41
+GP42,GPIO42
+GP43,GPIO43
+GP44,GPIO44
+GP45,GPIO45
+GP46,GPIO46
+GP47,GPIO47
+LED,GPIO25

firmware/README.md

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+# firmware
+
+Custom micropython build for the Olimex RP2350B-XL. Comes with some features enabled and modules bundled that are not part of an eventual upstream build.
+
+## build instructions
+
+0. Make sure to have a compiler toolchain supporting the RP2 architecture installed (Mac: `brew install gcc-arm-embedded`)
+1. Clone the `micropython` repo
+2. From within the `micropython` repo root, run `make -C mpy-cross`
+3. Copy the `OLIMEX_PICO2_XL` folder to `micropython/ports/rp2/boards` if it doesn't exist; overwrite it if it does
+4. Change to the `boards/rp2` folder
+5. Run `make BOARD=OLIMEX_PICO2_XL submodules` to install dependencies
+6. Run `make BOARD=OLIMEX_PICO2_XL clean` to remove previous build artefacts (if any)
+7. Run `make BOARD=OLIMEX_PICO2_XL` to build the firmware
+
+## flashing
+
+0. The build output can be found in `micropython/ports/rp2/build-OLIMEX_PICO2_XL` (or in this folder for convenience)
+1. Connect the MCU board to your computer using a data-capable USB cable
+2. On the MCU board, hold `boot` while pressing `rst`
+3. A new removable disk drive should appear. Copy `firmware.uf2` onto this disk drive - this will flash the MCU board with the custom micropython build.
+
+## using
+
+Once micropython is flashed, copy `boot.py`, `main.py` and any auxiliary files for the specific hardware type onto the MCU flash. Refer to the micropython docs for further info.