initial buildroot for linux 5.15

fs/initramfs/Config.in

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+config BR2_TARGET_ROOTFS_INITRAMFS
+	bool "initial RAM filesystem linked into linux kernel"
+	depends on BR2_LINUX_KERNEL
+	select BR2_TARGET_ROOTFS_CPIO
+	help
+	  Integrate the root filesystem generated by Buildroot as an
+	  initramfs inside the kernel image. This integration will
+	  take place automatically.
+
+	  A rootfs.cpio file will be generated in the images/ directory.
+	  This is the archive that will be included in the kernel image.
+	  The default rootfs compression set in the kernel configuration
+	  is used, regardless of how buildroot's cpio archive is
+	  configured.
+
+	  Note that enabling initramfs together with another filesystem
+	  formats doesn't make sense: you would end up having two
+	  identical root filesystems, one embedded inside the kernel
+	  image, and one separately.
+
+comment "initramfs needs a Linux kernel to be built"
+	depends on !BR2_LINUX_KERNEL

fs/initramfs/initramfs.mk

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+################################################################################
+#
+# Build a kernel with an integrated initial ramdisk filesystem based on cpio.
+#
+################################################################################
+
+# The generic fs infrastructure isn't very useful here.
+#
+# The initramfs image does not actually build an image; its only purpose is:
+# 1- to ensure rootfs.cpio is generated,
+# 2- to then rebuild the kernel with rootfs.cpio as initramfs
+#
+# Note: ordering of the dependencies is not guaranteed here, but in
+# linux/linux.mk, via the linux-rebuild-with-initramfs rule, which depends
+# on the rootfs-cpio filesystem rule.
+#
+# Note: the trick here is that we directly depend on rebuilding the Linux
+# kernel image (which itself depends on the rootfs-cpio rule), while we
+# advertise that our dependency is on the rootfs-cpio rule, which is
+# cleaner in the dependency graph.
+
+rootfs-initramfs: linux-rebuild-with-initramfs
+
+rootfs-initramfs-show-depends:
+	@echo rootfs-cpio
+
+.PHONY: rootfs-initramfs rootfs-initramfs-show-depends
+
+ifeq ($(BR2_TARGET_ROOTFS_INITRAMFS),y)
+TARGETS_ROOTFS += rootfs-initramfs
+endif
+
+# Not using the rootfs infra, so fake the variables
+ROOTFS_INITRAMFS_NAME = rootfs-initramfs
+ROOTFS_INITRAMFS_TYPE = rootfs
+ROOTFS_INITRAMFS_DEPENDENCIES = rootfs-cpio linux