zip-image: factor out with common

Notably, only include the strictly necessary files in the archive, otherwise
initrd's from other builds might get in the way.

zip-img

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 . "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/common"
 common_setup
+cd "$common_root_dir"
 outfile="${common_out_dir}/lkmc-${common_sha}.zip"
 rm -f "$outfile"
-for arch in x86_64 arm aarch64; do
+for common_arch in x86_64 arm aarch64; do
   common_setup
-  rm -f "${common_images_dir}/rootfs.ext2"
-  zip -r "$outfile" "$common_images_dir"
+  zip -r "$outfile" "${common_qcow2_file#${common_root_dir}/}" "${common_linux_image#${common_root_dir}/}"
 done