rp2/boards: Add board definition for Waveshare RP2040-Zero.

Signed-off-by: Oliver Joos <oliver.joos@hispeed.ch>

ports/rp2/boards/WAVESHARE_RP2040_ZERO/board.json

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+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "Dual-core",
+        "External Flash",
+        "USB"
+    ],
+    "images": [
+        "rp2040-zero.jpg"
+    ],
+    "mcu": "rp2040",
+    "product": "RP2040-Zero",
+    "thumbnail": "",
+    "url": "https://www.waveshare.com/wiki/RP2040-Zero",
+    "vendor": "Waveshare"
+}

ports/rp2/boards/WAVESHARE_RP2040_ZERO/board.md

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+This board has 2 MiB of flash memory, of which 1.5 MiB can be used as a USB
+drive for your own Python modules.
+
+The side pins are placed at a breadboard-friendly distance. Pin 16 is connected
+to a Neopixel RGB LED. Pins 17-25 are connected to small soldering pads at the
+bottom.

ports/rp2/boards/WAVESHARE_RP2040_ZERO/mpconfigboard.cmake

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+# cmake file for RP2040-Zero board
+set(PICO_BOARD "waveshare_rp2040_zero")

ports/rp2/boards/WAVESHARE_RP2040_ZERO/mpconfigboard.h

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+// Board and hardware specific configuration
+#define MICROPY_HW_BOARD_NAME           "RP2040-Zero"
+#define MICROPY_HW_FLASH_STORAGE_BYTES  (1536 * 1024)
+
+// Enable USB Mass Storage with FatFS filesystem.
+#define MICROPY_HW_USB_MSC (1)

ports/rp2/boards/WAVESHARE_RP2040_ZERO/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
+GP23,GPIO23
+GP24,GPIO24
+GP25,GPIO25
+GP26,GPIO26
+GP27,GPIO27
+GP28,GPIO28
+GP29,GPIO29
+NEOPIXEL,GPIO16