Add Makefile.

This will both improve CI, and decouple the kernel build script from
cookbook.

Makefile

+export RUST_TARGET_PATH=targets
+
+ifeq ($(TARGET),)
+	ARCH?=$(shell uname -a)
+else
+	ARCH?=$(shell echo "$(TARGET)" | cut -d - -f1)
+endif
+
+BUILD?=target/$(ARCH)-unknown-kernel
+
+all: $(BUILD)/kernel $(BUILD)/kernel.sym
+
+LD_SCRIPT=linkers/$(ARCH).ld
+TARGET_SPEC=targets/$(ARCH)-unknown-kernel.json
+
+$(BUILD)/kernel.all: $(LD_SCRIPT) $(TARGET_SPEC) $(shell find . -name "*.rs" -type f)
+	cargo rustc \
+		--bin kernel \
+		--target "$(TARGET_SPEC)" \
+		--release \
+		-Z build-std=core,alloc \
+		-- \
+		-C link-arg=-T -Clink-arg="$(LD_SCRIPT)" \
+		-C link-arg=-z -Clink-arg=max-page-size=0x1000 \
+		--emit link="$(BUILD)/kernel.all"
+
+$(BUILD)/kernel.sym: $(BUILD)/kernel.all
+	$(TARGET)-objcopy \
+		--only-keep-debug \
+		"$(BUILD)/kernel.all" \
+		"$(BUILD)/kernel.sym"
+
+$(BUILD)/kernel: $(BUILD)/kernel.all
+	$(TARGET)-objcopy \
+		--strip-debug \
+		"$(BUILD)/kernel.all" \
+		"$(BUILD)/kernel"