build: add package target

`make package` will produce a tarball containing the kernel image,
initramfs image, Raspberry Pi firmware, and root filesystem image.

Makefile

@@ -1,14 +1,29 @@
 BUILDROOT_SRC ?= ~/src/buildroot
 
-.PHONY: all initramfs rootfs
+.PHONY: all initramfs package rootfs
 
 all: initramfs rootfs
 
 rootfs: _build/rootfs/.config
 	$(MAKE) -C _build/rootfs
 
-initramfs: _build/initramfs/.config
+initramfs: _build/initramfs/images/initramfs.img.lz4
+
+package:
+	tar -c -f _build/metricspi.tar \
+		-C ${PWD}/_build/initramfs/images \
+		initramfs.img.lz4 \
+		-C ${PWD}/_build/rootfs/images \
+		rpi-firmware/ \
+		bcm2711-rpi-cm4.dtb \
+		Image \
+		rootfs.squashfs \
+		--
+
+_build/initramfs/images/initramfs.img.lz4: _build/initramfs/.config
 	$(MAKE) -C _build/initramfs
+	cd _build/initramfs/images && ln rootfs.cpio.lz4 initramfs.img.lz4
+
 
 _build/rootfs/.config: configs/metricspi_defconfig
 	$(MAKE) -C $(BUILDROOT_SRC) O=${PWD}/_build/rootfs BR2_EXTERNAL=${PWD} metricspi_defconfig