Add BR2_EXTERNAL skeleton

configs/rpi_common_test_defconfig

@@ -0,0 +1,36 @@
+# Architecture
+BR2_aarch64=y
+
+# Toolchain
+BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
+
+# Kernel
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_TARBALL=y
+BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="$(call github,raspberrypi,linux,stable_20231123)/linux-stable_20231123.tar.gz"
+BR2_LINUX_KERNEL_DEFCONFIG="bcm2711"
+BR2_LINUX_KERNEL_DTS_SUPPORT=y
+BR2_LINUX_KERNEL_INTREE_DTS_NAME="broadcom/bcm2711-rpi-4-b"
+BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
+
+# Bootloader
+BR2_PACKAGE_RPI_FIRMWARE=y
+BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI4=y
+
+# Filesystem
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_4=y
+
+# Host tools needed for image generation
+BR2_PACKAGE_HOST_GENIMAGE=y
+BR2_PACKAGE_HOST_DOSFSTOOLS=y
+BR2_PACKAGE_HOST_MTOOLS=y
+
+# Rootfs overlay and scripts from this BR2_EXTERNAL
+BR2_ROOTFS_OVERLAY="$(BR2_EXTERNAL_RPI_COMMON_PATH)/board/raspberrypi/overlay"
+BR2_ROOTFS_POST_BUILD_SCRIPT="$(BR2_EXTERNAL_RPI_COMMON_PATH)/board/raspberrypi/post_build.sh"
+BR2_ROOTFS_POST_IMAGE_SCRIPT="$(BR2_EXTERNAL_RPI_COMMON_PATH)/board/raspberrypi/post_image.sh"
+
+# Enable all common packages for testing
+BR2_PACKAGE_MY_SHARED_LIB=y
+BR2_PACKAGE_MY_COMMON_TOOL=y