package: Include Pi firmware in archive root

Raspberry Pi firmware files are now included in the root of the
distribution archive, insteaad of in an `rpi-firmware` subdirectory.
This will make it easier to extract them to the FAT32 partition on the
SD card, along with the kernel and initramfs images files.

Makefile

@@ -11,14 +11,15 @@ initramfs: _build/initramfs/images/initramfs.img.lz4
 
 package:
 	tar -c -f _build/metricspi.tar \
+		-C ${PWD}/_build/rootfs/images \
+		rootfs.squashfs \
 		-C ${PWD}/_build/initramfs/images \
 		initramfs.img.lz4 \
 		-C ${PWD}/_build/rootfs/images \
-		rpi-firmware/ \
 		bcm2711-rpi-cm4.dtb \
-		Image \
-		rootfs.squashfs \
-		--
+		Image
+	cd _build/rootfs/images/rpi-firmware && \
+	tar -r -f ${PWD}/_build/metricspi.tar *
 
 _build/initramfs/images/initramfs.img.lz4: _build/initramfs/.config
 	$(MAKE) -C _build/initramfs