initial buildroot for linux 5.15

board/friendlyarm/nanopi-m4/extlinux.conf

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+label RK3399_NANOPI_M4 linux
+  kernel /Image
+  devicetree /rk3399-nanopi-m4.dtb
+  append earlycon=uart8250,mmio32,0xff1a0000 root=/dev/mmcblk1p4 rw rootwait

board/friendlyarm/nanopi-m4/genimage.cfg

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+image boot.vfat {
+	vfat {
+		files = {
+			"Image",
+			"rk3399-nanopi-m4.dtb",
+			"extlinux"
+		}
+	}
+
+	size = 64M
+}
+
+image sdcard.img {
+	hdimage {
+		gpt = true
+	}
+
+	partition loader1 {
+		image = "idbloader.img"
+		offset = 32K
+	}
+
+	partition loader2 {
+		image = "u-boot.itb"
+		offset = 8M
+	}
+
+	partition boot {
+		partition-type = 0xC
+		bootable = "true"
+		image = "boot.vfat"
+		offset = 16M
+	}
+
+	partition rootfs {
+		partition-type = 0x83
+		image = "rootfs.ext4"
+	}
+}

board/friendlyarm/nanopi-m4/post-build.sh

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+#!/bin/sh
+
+BOARD_DIR="$(dirname $0)"
+
+install -m 0644 -D $BOARD_DIR/extlinux.conf $BINARIES_DIR/extlinux/extlinux.conf

board/friendlyarm/nanopi-m4/readme.txt

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+FriendlyARM NANOPI-M4
+=====================
+
+Build:
+
+  $ make friendlyarm_nanopi_m4_defconfig
+  $ make
+
+Files created in output directory
+=================================
+
+output/images
+
+├── bl31.elf
+├── boot.vfat
+├── extlinux
+├── idbloader.img
+├── Image
+├── rk3399-nanopi-m4.dtb
+├── rootfs.ext2
+├── rootfs.ext4 -> rootfs.ext2
+├── rootfs.tar
+├── sdcard.img
+├── u-boot.bin
+└── u-boot.itb
+
+Creating bootable SD card:
+==========================
+
+Simply invoke (as root)
+
+sudo dd if=output/images/sdcard.img of=/dev/sdX && sync
+
+Where X is your SD card device
+
+Serial console
+--------------
+
+Baudrate for this board is 1500000
+
+Login:
+------
+Enter 'root' as login user, and the prompt is ready.
+
+https://wiki.amarulasolutions.com/bsp/rockchip/rk3399/nanopi_m4.html