samd/boards: Add ADAFRUIT_FEATHER_M4_EXPRESS and _ITSYBITSY_M0_EXPRESS.

These two boards are used for testing, so it is favorable to have them
added early.

The full test set is:
- ADAFRUIT_FEATHER_M4_EXPRESS: SAMD51 with 32kHz crystal.
- ADAFRUIT_ITSYBITSY_M0_EXPRESS: SAMD21 without crystal.
- ADAFRUIT_ITSYBITSY_M4_EXPRESS: SAMD51 without crystal.
- SEEED_XIAO: SAM21 with 32kHz crystal.

ports/samd/boards/ADAFRUIT_FEATHER_M4_EXPRESS/board.json

@@ -0,0 +1,22 @@
+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "Battery Charging",
+        "Breadboard Friendly",
+        "Feather",
+        "Micro USB",
+        "RGB LED",
+        "SPI Flash"
+    ],
+    "images": [
+        "feather_m4_express.jpg"
+    ],
+    "mcu": "samd51",
+    "product": "Feather M4 Express",
+    "thumbnail": "",
+    "url": "https://www.adafruit.com/product/3857",
+    "vendor": "Adafruit"
+}

ports/samd/boards/ADAFRUIT_FEATHER_M4_EXPRESS/mpconfigboard.h

@@ -0,0 +1,2 @@
+#define MICROPY_HW_BOARD_NAME "Feather M4 Express"
+#define MICROPY_HW_MCU_NAME   "SAMD51J19A"

ports/samd/boards/ADAFRUIT_FEATHER_M4_EXPRESS/mpconfigboard.mk

@@ -0,0 +1,8 @@
+MCU_SERIES = SAMD51
+CMSIS_MCU = SAMD51J19A
+LD_FILES = boards/samd51j19a.ld sections.ld
+TEXT0 = 0x4000
+
+# The ?='s allow overriding in mpconfigboard.mk.
+# MicroPython settings
+MICROPY_VFS_LFS1 ?= 1

ports/samd/boards/ADAFRUIT_FEATHER_M4_EXPRESS/pins.csv

@@ -0,0 +1,34 @@
+# Pin rows contain Pin number and pin name.
+# Pin rows start with PIN_
+# LED rows start with LED_
+# If the pin name is omitted, the pin number is added as name.
+# Rows for empty entries have to start with '-'
+# Empty lines and lines starting with PIN_ or LED_ are ignored
+
+PIN_PB17,D0
+PIN_PB16,D1
+PIN_PA14,D4
+PIN_PA16,D5
+PIN_PA18,D6
+-
+PIN_PB03,D8
+PIN_PA19,D9
+PIN_PA20,D10
+PIN_PA21,D11
+PIN_PA22,D12
+PIN_PA23,D13
+PIN_PA02,A0
+PIN_PA05,A1
+PIN_PB08,A2
+PIN_PB09,A3
+PIN_PA04,A4
+PIN_PB06,A5
+PIN_PA13,SCL
+PIN_PA12,SDA
+PIN_PB23,MOSI
+PIN_PB22,MISO
+PIN_PA17,SCK
+PIN_PB01,VDIV
+PIN_PA03,AREF
+
+LED_PA17,LED

ports/samd/boards/ADAFRUIT_ITSYBITSY_M0_EXPRESS/board.json

@@ -0,0 +1,20 @@
+{
+    "deploy": [
+        "../deploy.md"
+    ],
+    "docs": "",
+    "features": [
+        "Breadboard Friendly",
+        "Micro USB",
+        "RGB LED",
+        "SPI Flash"
+    ],
+    "images": [
+        "itsybitsy_m0_express.jpg"
+    ],
+    "mcu": "samd21",
+    "product": "ItsyBitsy M0 Express",
+    "thumbnail": "",
+    "url": "https://www.adafruit.com/product/3727",
+    "vendor": "Adafruit"
+}

ports/samd/boards/ADAFRUIT_ITSYBITSY_M0_EXPRESS/mpconfigboard.h

@@ -0,0 +1,2 @@
+#define MICROPY_HW_BOARD_NAME "ItsyBitsy M0 Express"
+#define MICROPY_HW_MCU_NAME   "SAMD21G18A"

ports/samd/boards/ADAFRUIT_ITSYBITSY_M0_EXPRESS/mpconfigboard.mk

@@ -0,0 +1,8 @@
+MCU_SERIES = SAMD21
+CMSIS_MCU = SAMD21G18A
+LD_FILES = boards/samd21x18a.ld sections.ld
+TEXT0 = 0x2000
+
+# The ?='s allow overriding in mpconfigboard.mk.
+# MicroPython settings
+MICROPY_VFS_LFS1 ?= 1

ports/samd/boards/ADAFRUIT_ITSYBITSY_M0_EXPRESS/pins.csv

@@ -0,0 +1,40 @@
+# Pin rows contain Pin number and pin name.
+# Pin rows start with PIN_
+# LED rows start with LED_
+# If the pin name is omitted, the pin number is added as name.
+# Rows for empty entries have to start with '-'
+# Empty lines and lines not starting with PIN_ or LED_ are ignored
+
+PIN_PA11,D0
+PIN_PA10,D1
+PIN_PA14,D2
+PIN_PB09,D3
+PIN_PA08,D4
+PIN_PA15,D5
+-
+PIN_PA21,D7
+-
+PIN_PA07,D9
+PIN_PA18,D10
+PIN_PA16,D11
+PIN_PA19,D12
+PIN_PA17,D13
+PIN_PA02,A0
+PIN_PB08,A1
+PIN_PB09,A2
+PIN_PA04,A3
+PIN_PA05,A4
+PIN_PB02,A5
+PIN_PA22,SDA
+PIN_PA23,SCL
+PIN_PB10,MO
+PIN_PA12,MI
+PIN_PB11,SCK
+PIN_PA00,DOTSTAR_CLK
+PIN_PA01,DOTSTAR_DATA
+PIN_PB22,FLASH_MOSI
+PIN_PB03,FLASH_MISO
+PIN_PB23,FLASH_SCK
+PIN_PA27,FLASH_CS
+
+LED_PA17,LED

ports/samd/boards/samd51j19a.ld

@@ -0,0 +1,17 @@
+/*
+    GNU linker script for SAMD51
+*/
+
+/* Specify the memory areas */
+MEMORY
+{
+    FLASH (rx)  : ORIGIN = 0x00004000, LENGTH = 512K - 16K
+    RAM (xrw)   : ORIGIN = 0x20000000, LENGTH = 192K
+}
+
+/* Top end of the stack, with room for double-tap variable */
+_estack = ORIGIN(RAM) + LENGTH(RAM) - 8;
+_sstack = _estack - 16K;
+
+_sheap = _ebss;
+_eheap = _sstack;