Add Milk-V DuoS, an aarch64+riscv64 board

config/boards/milkv-duos-arm.csc

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+# Sophgo SG2000 Cortex-A53 single core 512MB SoC headless 1x100MBe SD eMMC WiFi/BT USB2
+BOARD_NAME="Milk-V Duo S"
+BOARD_VENDOR="milkv"
+BOARDFAMILY="sophgo-sg200x-arm64"
+BOARD_MAINTAINER="lukaszsobala"
+INTRODUCED="2024"
+KERNEL_TARGET="edge,bleedingedge"
+KERNEL_TEST_TARGET="edge"
+BOOT_FDT_FILE="sophgo/sg2000-milkv-duo-s.dtb"
+HAS_VIDEO_OUTPUT="no"
+PACKAGE_LIST_BOARD_REMOVE="snapd cloud-init" # 512MB of RAM
+
+enable_extension "sophgo-sg200x-aic8800"

config/boards/milkv-duos-riscv.csc

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+# Sophgo SG2000 T-Head C906 single core 512MB SoC headless 1x100MBe SD eMMC WiFi/BT USB2
+BOARD_NAME="Milk-V Duo S"
+BOARD_VENDOR="milkv"
+BOARDFAMILY="sophgo-sg200x-riscv64"
+BOARD_MAINTAINER="lukaszsobala"
+INTRODUCED="2024"
+KERNEL_TARGET="edge,bleedingedge"
+KERNEL_TEST_TARGET="edge"
+BOOT_FDT_FILE="sophgo/sg2000-milkv-duo-s.dtb"
+HAS_VIDEO_OUTPUT="no"
+PACKAGE_LIST_BOARD_REMOVE="snapd cloud-init" # 512MB of RAM
+
+enable_extension "sophgo-sg200x-aic8800"

config/bootenv/sophgo-sg200x-arm64.txt

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+verbosity=1
+console=serial
+earlycon=on
+overlays=

config/bootenv/sophgo-sg200x-riscv64.txt

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+verbosity=1
+console=serial
+earlycon=sbi
+overlays=

config/bootscripts/boot-sophgo-sg200x.cmd

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+# DO NOT EDIT THIS FILE
+#
+# Please edit /boot/armbianEnv.txt to set supported parameters
+#
+
+# Scratch space for armbianEnv.txt and for each overlay before it is applied.
+# 0x82300000 is fdtoverlay_addr_r from the vendor U-Boot's distroboot patch: it
+# sits between fdt_addr_r (0x82200000) and pxefile_addr_r (0x82400000), so there
+# is 1MiB for the device tree to grow into and 1MiB of scratch after it.
+setenv load_addr "0x82300000"
+setenv overlay_error "false"
+
+# Defaults, overridden by armbianEnv.txt below. The Duo S has no display, so the
+# console is serial-only; "both" would add a console=tty1 that never appears.
+setenv rootdev "/dev/mmcblk0p2"
+setenv verbosity "1"
+setenv console "serial"
+setenv rootfstype "ext4"
+setenv earlycon "off"
+
+test -n "${distro_bootpart}" || distro_bootpart=1
+
+echo "Boot script loaded from ${devtype} ${devnum}:${distro_bootpart}"
+
+if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}armbianEnv.txt; then
+	load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}armbianEnv.txt
+	env import -t ${load_addr} ${filesize}
+fi
+
+if test "${console}" = "display" || test "${console}" = "both"; then setenv consoleargs "console=tty1"; fi
+if test "${console}" = "serial" || test "${console}" = "both"; then setenv consoleargs "console=ttyS0,115200 ${consoleargs}"; fi
+
+# Two spellings, because the two architectures need different ones and share
+# this script. The C906 has no memory-mapped earlycon before the SBI console is
+# up, so RISC-V wants earlycon=sbi; on the Cortex-A53 the device tree's
+# stdout-path is enough and plain earlycon works.
+if test "${earlycon}" = "on"; then setenv consoleargs "earlycon ${consoleargs}"; fi
+if test "${earlycon}" = "sbi"; then setenv consoleargs "earlycon=sbi ${consoleargs}"; fi
+
+setenv bootargs "root=${rootdev} rootwait rootfstype=${rootfstype} ${consoleargs} rw no_console_suspend consoleblank=0 loglevel=${verbosity} fsck.fix=yes fsck.repair=yes net.ifnames=0 ${extraargs} ${extraboardargs}"
+
+load ${devtype} ${devnum}:${distro_bootpart} ${ramdisk_addr_r} ${prefix}uInitrd
+load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} ${prefix}Image
+
+load ${devtype} ${devnum}:${distro_bootpart} ${fdt_addr_r} ${prefix}dtb/${fdtfile}
+fdt addr ${fdt_addr_r}
+fdt resize 65536
+for overlay_file in ${overlays}; do
+	if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}dtb/sophgo/overlay/${overlay_prefix}-${overlay_file}.dtbo; then
+		echo "Applying kernel provided DT overlay ${overlay_prefix}-${overlay_file}.dtbo"
+		fdt apply ${load_addr} || setenv overlay_error "true"
+	fi
+done
+for overlay_file in ${user_overlays}; do
+	if load ${devtype} ${devnum}:${distro_bootpart} ${load_addr} ${prefix}overlay-user/${overlay_file}.dtbo; then
+		echo "Applying user provided DT overlay ${overlay_file}.dtbo"
+		fdt apply ${load_addr} || setenv overlay_error "true"
+	fi
+done
+if test "${overlay_error}" = "true"; then
+	echo "Error applying DT overlays, restoring original DT"
+	load ${devtype} ${devnum}:${distro_bootpart} ${fdt_addr_r} ${prefix}dtb/${fdtfile}
+fi
+
+booti ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr_r}
+
+# Recompile with:
+# mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr