coreos/stage1_hooks: Add a hook for updating a profile in stage1

This is to make sure that the directory layout wrt. lib directories in
stage1 is correctly set up from the beginning, because it gets
propagated all the way to the final SDK image. It's easier to do it
that way, rather than following the steps described in the deprecation
notice of the 17.0 profile.

sdk_container/src/third_party/coreos-overlay/coreos/stage1_hooks/0001-update-profile-coreos-overlay.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -x
+set -euo pipefail
+
+stage1_repo="${1}"
+new_repo="${2}"
+parent_file='profiles/coreos/amd64/parent'
+old_parent_line='portage-stable:default/linux/amd64/17.0/no-multilib/hardened'
+stage1_parent="${stage1_repo}/${parent_file}"
+new_parent="${new_repo}/${parent_file}"
+
+if [[ ! -e "${new_parent}" ]]; then
+    echo "no file '${parent_file}' in new repo, nothing to do"
+    exit 0
+fi
+
+if [[ ! -e "${stage1_parent}" ]]; then
+    echo "no file '${parent_file}' in stage1 repo, nothing to do"
+    exit 0
+fi
+
+if grep --quiet --fixed-strings --line-regexp --regexp="${old_parent_line}" -- "${stage1_parent}"; then
+    rm -f "${stage1_parent}"
+    cp -a "${new_parent}" "${stage1_parent}"
+fi