Add buildroot for building test kernel/initramfs

.dockerignore

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+output/
+buildroot/

.gitignore

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+output/

.gitmodules

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+[submodule "buildroot"]
+	path = buildroot
+	url = https://github.com/buildroot/buildroot.git

Makefile

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+.DEFAULT_GOAL := test-images
+
+OUTPUT_ROOT=output/
+TEST_GUEST_KERNEL_TARGET=$(OUTPUT_ROOT)/test-bzImage
+TEST_GUEST_KERNEL_INITRAMFS=$(OUTPUT_ROOT)/test-initramfs.img
+
+.PHONY: test-images
+test-images: $(TEST_GUEST_KERNEL_TARGET) $(TEST_GUEST_KERNEL_INITRAMFS)
+
+$(OUTPUT_ROOT):
+	mkdir -p $(OUTPUT_ROOT)
+
+$(TEST_GUEST_KERNEL_TARGET): buildroot.config kernel.config $(OUTPUT_ROOT)
+	cp buildroot.config buildroot/.config
+	make -C buildroot linux
+	cp buildroot/output/images/bzImage $(TEST_GUEST_KERNEL_TARGET)
+
+$(TEST_GUEST_KERNEL_INITRAMFS): $(TEST_GUEST_KERNEL_TARGET)
+	cp buildroot.config buildroot/.config
+	make -C buildroot
+	cp buildroot/output/images/rootfs.cpio.gz $(TEST_GUEST_KERNEL_INITRAMFS)
+
+.PHONY: docker
+docker:
+	docker build .
+
+.PHONY: clean
+clean:
+	rm -rf $(OUTPUT_ROOT)
+	make -C buildroot clean