Add BR2_EXTERNAL skeleton

board/raspberrypi/genimage.cfg

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+/* genimage.cfg - SD card image layout for Raspberry Pi
+ *
+ * Partition layout:
+ *   - boot (FAT32, 64MB): kernel, device tree, bootloader files
+ *   - rootfs (ext4, 256MB): root filesystem
+ *
+ * The final image is written to sdcard.img.
+ *
+ * Adjust partition sizes as needed for your project. Projects can
+ * override this file by providing their own genimage.cfg and setting
+ * GENIMAGE_CFG in their post-image script.
+ */
+
+image boot.vfat {
+    vfat {
+        files = {
+            "bcm2710-rpi-3-b.dtb",
+            "bcm2711-rpi-4-b.dtb",
+            "rpi-firmware/bootcode.bin",
+            "rpi-firmware/start.elf",
+            "rpi-firmware/fixup.dat",
+            "rpi-firmware/config.txt",
+            "rpi-firmware/cmdline.txt",
+            "Image"
+        }
+    }
+
+    size = 64M
+}
+
+image rootfs.ext4 {
+    ext4 {
+        label = "rootfs"
+    }
+
+    size = 256M
+}
+
+image sdcard.img {
+    hdimage {
+    }
+
+    partition boot {
+        partition-type = 0xC
+        bootable = "true"
+        image = "boot.vfat"
+    }
+
+    partition rootfs {
+        partition-type = 0x83
+        image = "rootfs.ext4"
+    }
+}

board/raspberrypi/overlay/etc/network/interfaces

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+# /etc/network/interfaces - Common network configuration
+#
+# This file is part of the shared rootfs overlay and provides a
+# sensible default network configuration for all Raspberry Pi projects.
+# Projects can override this by placing their own version in their
+# project-specific overlay (which is applied after this one).
+
+auto lo
+iface lo inet loopback
+
+auto eth0
+iface eth0 inet dhcp

board/raspberrypi/post_build.sh

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+#!/bin/sh
+# post_build.sh - Common post-build script for Raspberry Pi
+#
+# Called by Buildroot after assembling the target rootfs.
+# Arguments:
+#   $1 = path to the target rootfs directory (e.g., output/target)
+#
+# Environment:
+#   BR2_EXTERNAL_RPI_COMMON_PATH = path to this external tree
+#
+# Use this script for:
+#   - Fixing file permissions
+#   - Creating symlinks
+#   - Generating config files that depend on build-time variables
+#   - Stripping unnecessary files from the rootfs
+#
+# Do NOT use this for:
+#   - Installing packages (use rootfs overlay or a package .mk instead)
+#   - Modifying files outside $1
+
+set -e
+
+TARGET_DIR="$1"
+
+# Example: ensure /etc/hostname exists with a default value
+if [ ! -f "${TARGET_DIR}/etc/hostname" ]; then
+    echo "rpi-common" > "${TARGET_DIR}/etc/hostname"
+fi
+
+# Example: remove unnecessary documentation to save space
+rm -rf "${TARGET_DIR}/usr/share/man"
+rm -rf "${TARGET_DIR}/usr/share/doc"
+
+echo ">>> Common post-build script completed"

board/raspberrypi/post_image.sh

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+#!/bin/sh
+# post_image.sh - Common post-image script for Raspberry Pi
+#
+# Called by Buildroot after filesystem images (ext4, etc.) are generated.
+# Arguments:
+#   $1 = path to the images directory (e.g., output/images)
+#
+# Environment:
+#   BR2_EXTERNAL_RPI_COMMON_PATH = path to this external tree
+#   BINARIES_DIR                 = same as $1
+#   BUILD_DIR                    = path to the build directory
+#
+# This script uses genimage to assemble the final SD card image
+# from the boot files and root filesystem.
+
+set -e
+
+BOARD_DIR="${BR2_EXTERNAL_RPI_COMMON_PATH}/board/raspberrypi"
+
+# Use project-specific genimage.cfg if it exists, otherwise use common one.
+# Projects can override by setting GENIMAGE_CFG in their own post-image script.
+GENIMAGE_CFG="${GENIMAGE_CFG:-${BOARD_DIR}/genimage.cfg}"
+
+# genimage requires a temporary directory
+GENIMAGE_TMP="${BUILD_DIR}/genimage.tmp"
+rm -rf "${GENIMAGE_TMP}"
+
+genimage \
+    --rootpath "${TARGET_DIR}" \
+    --tmppath "${GENIMAGE_TMP}" \
+    --inputpath "${BINARIES_DIR}" \
+    --outputpath "${BINARIES_DIR}" \
+    --config "${GENIMAGE_CFG}"
+
+echo ">>> SD card image generated: ${BINARIES_DIR}/sdcard.img"