AB: Pack slot archive in post-build

Author: learmj

image/gpt/ab_userdata/post-image.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+set -eu
+
+upd="${1}/update"
+rm -rf $upd
+mkdir -p $upd
+
+[[ -f ${1}/system.sparse ]] || false
+[[ -f ${1}/boot.sparse ]] || false
+
+ln -sf ../system.sparse ${upd}/system
+ln -sf ../boot.sparse ${upd}/boot
+
+msg "Packing..."
+cd $upd && tar -I zstd -h -cf  ${1}/update.tar.zst -- *